We studied prospectively eight healthy postpartum breast-feeding women for 6 months during early, middle, and late lactation. Blood for measurement of oxytocin (OT), PRL, and arginine vasopressin was drawn before and every 3 min from each women during 15 min of infant suckling for two consecutive feedings during each stage of lactation. Basal plasma OT was not different in breast-feeding [0.7 +/- 0.1 (+/- SEM) microU/ml] and nonbreast-feeding women (0.8 +/- 0.2 microU/ml). OT increased significantly in response to infant suckling (P less than 0.00001) to 5.9 +/- 0.5 microU/ml and remained elevated throughout a feeding. OT was released during infant suckling in an episodic pattern in some, but not all, women; peak OT varied among women (5.0-23.3 microU/ml). There was no significant difference in the mean stimulated OT or the pattern of release comparing the first and second feedings of the same day. The mean OT (n = 8) released during 15 min of infant suckling was not significantly different in early (3.9 +/- 0.3 microU/ml), middle (4.5 +/- 0.3 microU/ml), and late (5.8 +/- 0.4 microU/ml) lactation. In the four women who breast fed exclusively, the mean stimulated OT was significantly higher (P less than 0.01) during late lactation (8.6 +/- 0.4 microU/ml) vs. early (4.6 +/- 0.4 microU/ml) or middle (6.1 +/- 0.4 microU/ml) lactation. In the other four women who provided formula supplements, OT did not change. Plasma arginine vasopressin did not increase in response to infant suckling. Plasma PRL increased in response to infant suckling, reaching a peak at 15 min. Mean basal PRL decreased progressively from weeks 1-24 postpartum. Mean peak PRL decreased significantly from early (162 +/- 29) to middle (130 +/- 15) to late (77 +/- 10) ng/ml lactation (P less than 0.05). OT release in response to infant suckling continues throughout the first 6 months postpartum in breast-feeding women, and the pattern is reproducible. The maximum release of OT is dependent upon continuous regular nipple stimulation. In contrast to basal and suckling-induced levels of PRL, which decreased with time postpartum, basal and suckling-induced OT release did not decrease from early to late lactation.