A study was made of diurnal trends in net photosynthetic rate and carbohydrate levels of unifoliolate leaves of soybean (Glycine max L. Merrill) under constant environmental conditions (50,000-lux light intensity, 24.5 C air temperature, 60% relative humidity, and 300 microliters of CO(2) per liter of air).Net photosynthetic rate remained relatively constant between 4 and 10 hours after the lights were turned on but then gradually declined to 85% of this rate by the end of the 16-hour photoperiod. The decline in net photosynthetic rate was due to increases in both stomatal diffusion resistance and residual resistance to CO(2).The decline in net photosynthetic rate began when the rate of starch accumulation began to decline rapidly. At this time, there also appeared to be an increase in soluble carbohydrate level. The results suggest that when a high starch level was reached, further starch synthesis was impaired, leading to an increase in soluble carbohydrate level and, consequently, a reduction in net photosynthetic rate.