The renal excretion of p-aminohippurate (PAH) can be stimulated by repeated administration of cyclopenthiazide (5 mg/100 g b.w., i.p. for 3 days, twice daily). The reason for this seems to be an increased renal tubular transport capacity for weak organic acids. Inhibitors of protein biosynthesis as azauracil, actinomycin D, neomycin and cycloheximide influence the cyclopenthiazide stimulated p-aminohippurate excretion. Azauracil (5 mg/100 g b.w.), an inhibitor of replication is effective in the same way as inhibitors acting on transcription and translation (actinomycin D 7.5-60 micrograms/100 g b.w.; neomycin 5-10 mg) 100 g b.w.; cycloheximide 0.02-0.05 mg/100 g b.w., i.p. for 4 days, once daily). Probably these substances inhibit the de novo synthesis of carrier proteins, increased by stimulation of renal tubular transport processes.