The presence of strong sinks of photoassimilates is thought to stimulate photosynthesis by minimizing photosynthetic end product accumulation in leaves. This hypothesis was examined in soybeans (Glycine max [L] Merr.) with treatments designed to alter the phloem translocation of photoassimilates out of source leaves. Pod removal and petiole girdling resulted in 70 and 90% reductions, respectively, in leaf CO(2) exchange rate. Reductions of similar magnitude also were observed in stomatal diffusive conductivity.Time course data showed that CO(2) exchange rates as well as stomatal conductivities were significantly reduced 0.5 hour after girdling and 5 hours after pod removal, thus suggesting that stomatal closure was the cause of the reduced rates of photosynthesis.Mesophyll conductivities to CO(2), calculated from gas exchange data, were not affected by the treatments. Radiocarbon assimilation rates by leaf slices floated in KH(14)CO(3) solutions were not reduced by pod removal and were reduced only 10% by girdling. It was concluded that treatments which block or slow translocation from source leaves reduce their photosynthetic rate by inducing stomatal closure.