The effect of nursing on plasma levels of oxytocin (OT), arginine vasopressin (AVP), and PRL was studied in six normal women 2-3 days post partum. Maternal blood samples were obtained for measurement of OT, AVP, PRL, sodium, and osmolality 3 and 0 min before suckling, at 3-min intervals for 15 min during suckling, and 5 min after completion of suckling. Plasma OT rose during suckling from a mean (+/- SEM) baseline value of 1.1 +/- 0.2 to 3.6 +/- 0.6 microU/ml by 3 min (P < 0.001), reached a peak level of 6.4 +/- 1.5 microU/ml by 6 min (P < 0.005), and remained elevated for the entire 15-min period of suckling. Serial measurements of plasma OT during suckling failed to show a pattern consistent with episodic secretion. The baseline plasma AVP concentration was 0.4 +/- 0.1 microU/ml and was not significantly altered by suckling. Plasma sodium and osmolality remained unchanged during the suckling period. The baseline serum PRL level was 268 +/- 24 ng/ml and rose to 362 +/- 31 ng/ml after 15 min of suckling (P < 0.05). The data suggest that suckling is a specific stimulus for OT and PRL secretion but has no effect on AVP release.