The slow-wilting soybean [Glycine max (L.) Merr.] genotype, PI 416937, exhibits a limiting leaf hydraulic conductance for transpiration rate (TR) under high vapour pressure deficit (VPD). This genotype has a constant TR at VPD greater than 2 kPa, which may be responsible for its drought tolerance as a result of soil water conservation. However, the exact source of the hydraulic limitation between symplastic and apoplastic water flow in the leaf under high VPD conditions are not known for PI 416937. A comparison was made in the TR response to aquaporin (AQP) inhibitors between PI 416937 and N01-11136, a commercial genotype that has a linear TR response to VPD in the 1-3.5 kPa range. Three AQP inhibitors were tested: cycloheximide (CHX, a de novo synthesis inhibitor), HgCl(2), and AgNO(3). Dose-response curves for the decrease in TR following exposure to each inhibitor were developed. Decreases in TR of N01-11136 following treatment with inhibitors were up to 60% for CHX, 82% for HgCl(2), and 42% for AgNO(3). These results indicate that the symplastic pathway terminating in the guard cells of these soybean leaves may be at least as important as the apoplastic pathway for water flow in the leaf under high VPD. While the decrease in TR for PI 416937 was similar to that of N01-11136 following exposure to CHX and HgCl(2), TR of PI 416937 was insensitive to AgNO(3) exposure. These results indicate the possibility of a lack of a Ag-sensitive leaf AQP population in the slow-wilting line, PI 416937, and the presence of such a population in the commercial line, N01-11136.