Inhibition of renal hippurate transport in vitro has been reported previously. Since many assessments of renal blood flow and tubule function are dependent on hippurate transport, the question of whether inhibition by substances present in azotemia occurs in vivo is important. These studies show that in azotemia, induced by two different methods, renal hippurate accumulation is inhibited in the kidney of rats in vivo. As in the in vitro model, oral neomycin feeding can reduce some of this inhibition. The similarity between the in vivo and in vitro models suggests that a common mechanism for inhibition of hippurate transport is present in both models.