Brattleboro rats were housed singly in metabolic cages and continuously infused with saline. Timed urine collections were analyzed for Na+ and K+; oxytocin (OT)-like material was estimated by RIA. OT infusions reduced urine flow and increased Na+ output. Urinary Na+/K+ ratios rose during OT infusion, falling below control values after infusion. Passive immunization with an OT antiserum reduced but did not prevent these renal effects of OT. In a second experiment, immunization had no effect on daily urine volume or Na+ output, even though high titers were achieved and excretion of endogenous OT-like material was reduced. We conclude that endogenous OT has little effect on Na+ excretion in these rats, although pharmacological doses of OT produced their known natriuretic effect.