Experiments were done in conscious, unrestrained, baroreceptor-intact rats to investigate the effects of afferent renal nerve (ARN) stimulation on circulating levels of arginine vasopressin (AVP) and oxytocin (OXY). Electrical stimulation of ARN elicited a rise in arterial pressure (AP) of 12 +/- 2 mmHg and no significant change in heart rate (HR). Plasma concentrations of AVP and OXY measured by radioimmunoassay were significantly increased immediately after the stimulation period. AVP increased from control levels of 1.89 +/- 0.72 to 4.59 +/- 1.19 pg/ml after stimulation of ARN and OXY increased from 4.3 +/- 0.35 to 10.0 +/- 1.00 pg/ml. AVP remained significantly elevated at 1 h after stimulation and gradually returned to control levels by 3 h after stimulation. On the other hand, OXY values were at control levels at 1 h after stimulation. Stimulation of ARN, after cutting ARN proximal to the stimulating electrode, and sham ARN stimulation did not alter AP, HR, or plasma levels of AVP and OXY. Plasma osmolality was not altered during the experiments. These data demonstrate that, in the conscious rat, afferent information from the kidney alters the release of AVP and OXY from the neurohypophysis and suggests that ARNs are important components of a neuronal circuit that modulates differentially the release of these hormones, which function in the homeostatic regulation of AP and fluid balance.