Abstract Ochratoxin A, a nephrotoxin produced as a secondary metabolite by A. ochraceus, is a potent inhibitor of renal PEPCK activity, in vivo. When fed orally to rats for 2 days, renal PEPCK activity is reduced 50% by a total dose of 0.3-0.5 mg toxin. Renal gluconeogenic capacity is reduced only after PEPCK activity is inhibited by 50%. Hepatic PEPCK activity is unaffected up to 1.5-2.0 mg ochratoxin A, which were the highest doses tested. Other enzymes located in proximal convoluted tubules, including phosphatedependent glutaminase, γ-glutamyl transpeptidase, pyruvate carboxylase, and Na,K-ATPase, are not affected. Renal protein synthesis from [3H]phenylalanine or [3H]leucine is inhibited 30–40% by ochratoxin A in vivo. By covalently coupling the toxin to albumin with carbodiimide or mixed anhydride, the inhibitory effect on renal PEPCK activity is retained, but protein synthesis is not affected and cytological evidence of nephrotoxicity is lost. Injection of the ochratoxin A-albumin carbodiimide complex results in a decrease of hepatic PEPCK activity as well. Removal of the phenylalanine group from the toxin prevents the in vivo inhibition of PEPCK activity, as well as protein synthesis. We conclude that the decrease in renal PEPCK activity, in vivo, requires the phenylalanine group of ochratoxin A, and occurs by a mechanism independent of the known nephrotoxicity effects.