Plasma levels of oxytocin and prolactin were measured before and during 12 min of breast pump stimulation in 5 healthy, lactating, amenorrheic women on 3 occasions: 10-90 days post partum, 90-180 days post partum, and 180 days to 1 yr post partum. Baseline mean (.+-. standard error of the mean) plasma oxytocin levels were similar in the 3 study periods. Mean stimulated plasma oxytocin levels increased in the 3 study periods (each P < 0.001; mean baseline vs. stimulated). Stimulated plasma oxytocin values were significantly greater at 10-90 than at 90-180 days (P < 0.05; analysis of variance). Baseline serum prolactin levels were 61 .+-. 9.5, 36 .+-. 8.6, and 33 .+-. 10.8 ng/ml, respectively, (not significant; 1-way analysis of variance). Mean stimulated prolactin levels were 71 .+-. 8.1, 43 .+-. 4.5, and 43 .+-. 2.8 ng/ml, respectively, (not significant). The oxytocin secretory reflex continues in long-term lactation for the 1st yr post partum. Breast stimulation in long-term lactating women continues to produce a slight increase in serum prolactin levels.