To quantify the renal excretion of arginine vasotocin (AVT) in birds, synthetic AVT was infused into conscious Pekin ducks at rates of 0.03, 0.10, and 0.30 ng/kg/min for 30 min and the relationships between plasma and urine AVT concentrations were monitored by radioimmunoassay. As plasma AVT concentrations increased (from a basal value of 7.8 +/- 1.1 pg/ml to levels of 11.5 +/- 1.9, 17.6 +/- 2.2, and 24.2 +/- 2.5 pg/ml, respectively) the urinary concentrations and excretion rates increased linearly from basal levels of 26.0 +/- 3.1 pg/ml and 14.9 +/- 1.8 pg/min to 215.6 +/- 36.6 pg/ml and 39.2 +/- 3.7 pg/min, respectively. The metabolic clearance rate of AVT (approximately 23 ml/min/kg) and urinary clearance (approximately 0.91 ml/min/kg) remained constant at each infusion dose and were therefore independent of plasma AVT levels. Over the range of plasma AVT concentrations tested, a constant fraction of about 5% of the AVT cleared from the blood was excreted intact in the urine.