1. The effect of arginine vasopressin (AVP) on ureagenesis was measured in isolated rat hepatocytes with ammonium chloride and l(+)-lactate as substrates.
2. AVP was found to stimulate urea synthesis and the dose-response curve suggests that such an effect is present at concentrations of the hormone as low as 25-50 pmol/l.
3. Both the dose-response curve and the concentrations of NH4+ employed suggest that the effect observed could be of physiological significance.