for customers. The additional costs incurred to perform these infrastructure and labor-intensive services inherently result in lower average gross margins as compared to maintenance and support services. Within the Services & Support segment, we do expect variability in gross margins from quarter-to-quarter based on the mix of the services recognized.
Gross Profit
As a percentage of revenue, gross profit decreased from 37.3% for the three months ended June 30, 2025, to 37.0% for the three months ended June 30, 2026, and increased from 37.9% for the six months ended June 30, 2025, to 38.2% for the six months ended June 30, 2026. The gross profit as a percentage of revenue for the three and six months ended June 30, 2026, remained relatively flat, with minor fluctuations driven by changes in customer and product mix. For the three and six months ended June 30, 2026, as compared to the three and six months ended June 30, 2025, changes in foreign currencies relative to the U.S. dollar increased our gross profit by approximately $0.7 million and $1.3 million, respectively.
As a percentage of that segment's revenue, Network Solutions gross profit decreased from 32.9% for the three months ended June 30, 2025, to 32.3% for the three months ended June 30, 2026, and increased from 33.2% for the six months ended June 30, 2025 to 33.7% for the six months ended June 30, 2026. The decrease in gross profit for the three and six months ended June 30, 2026, remained relatively flat, with minor fluctuations driven by changes in customer and product mix.
As a percentage of that segment's revenue, Services & Support gross profit increased from 58.7% for the three months ended June 30, 2025, to 59.4% for the three months ended June 30, 2026, and increased from 59.2% for the six months ended June 30, 2025, to 60.5% for the six months ended June 30, 2026. The increase in gross profit for the three and six months ended June 30, 2026 remained relatively flat, with minor fluctuations driven by cyclical changes in customer buying habits.
Selling, General and Administrative Expenses
As a percentage of revenue, selling, general and administrative expenses decreased from 22.8% for the three months ended June 30, 2025, to 21.4% for the three months ended June 30, 2026, and decreased from 21.6% for the six months ended June 30, 2025, to 20.5% for the six months ended June 30, 2026. Selling, general and administrative expenses as a percentage of revenue will generally fluctuate whenever there is a significant fluctuation in revenue for the periods being compared.
Selling, general and administrative expenses decreased 0.2% from $60.3 million for the three months ended June 30, 2025, to $60.2 million for the three months ended June 30, 2026, and increased 4.9% from $110.6 million for the six months ended June 30, 2025, to $116.1 million for the six months ended June 30, 2026. Selling, general and administrative expenses include personnel costs for management and back office departments, as well as auditor, tax and other professional fees. The increase for the six months ended June 30, 2026, compared to the six months ended June 30, 2025, was primarily attributable to increased employee-related costs and increased travel related expenses. For the three and six months ended June 30, 2026, as compared to the three and six months ended June 30, 2025, changes in foreign currencies relative to the U.S. dollar increased our selling, general and administrative expenses by approximately $0.6 million and $1.3 million, respectively.
Research and Development Expenses
As a percentage of revenue, research and development expenses decreased from 19.6% for the three months ended June 30, 2025, to 19.1% for the three months ended June 30, 2026, and decreased from 19.6% for the six months ended June 30, 2025, to 18.4% for the six months ended June 30, 2026. Research and development expenses as a percentage of revenue will generally fluctuate whenever there are incremental product development activities or significant fluctuations in revenue for the periods being compared.
Research and development expenses increased 3.6% from $51.9 million for the three months ended June 30, 2025, to $53.8 million for the three months ended June 30, 2026, and increased 3.8% from $100.8 million for the six months ended June 30, 2025, to $104.6 million for the six months ended June 30, 2026. The increase in research and development expenses for the three months ended June 30, 2026 compared to the three months ended June 30, 2025, was primarily attributable to increased employee-related costs. The increase in research and development expenses for the six months ended June 30, 2026 compared to the six months ended June 30, 2025, was primarily attributable to increased employee-related costs partially offset by increased governmental research and development subsidies. For the three and six months ended June 30, 2026 as compared to the three and six months ended June 30, 2025, changes in foreign currencies relative to the U.S. dollar increased our research and development expenses by approximately $1.3 million and $2.5 million, respectively.
Adtran Networks has arrangements with governmental entities for the purpose of obtaining funding for research and development activities. The Company classifies government grants received under these arrangements as a reduction to research and development expenses incurred. For the three months ended June 30, 2026 and 2025, the Company recognized $3.4 million and $3.1 million as a reduction of research and development expense, respectively. For the six months ended June 30, 2026 and 2025, the Company recognized $6.4 million and $5.3 million as a reduction of research and development expense, respectively.
We expect to continue to incur research and development expenses in connection with our new and existing products. We continually evaluate new product opportunities and engage in significant research and product development efforts, which provides for new product