Plasma levels of oxytocin and prolactin were measured before and during 12 minutes of breast pump stimulation in five healthy, lactating, amenorrheic women on three occasions: ten to 90 days post partum, 90 to 180 days post partum, and 180 days to one year post partum. Baseline mean (+/- SEM) plasma oxytocin levels were similar in the three study periods. Mean stimulated plasma oxytocin levels increased in the three study periods (each P less than .001; mean baseline versus stimulated). Stimulated plasma oxytocin values were significantly greater at ten to 90 than at 90 to 180 days (P less than .05; analysis of variance). Baseline serum prolactin levels were 61 +/- 9.5, 36 +/- 8.6, and 33 +/- 10.8 ng/ml, respectively (not significant; one-way analysis of variance). Mean stimulated prolactin levels were 71 +/- 8.1, 43 +/- 4.5, and 43 +/- 2.8 ng/ml, respectively (not significant). Thus, the oxytocin secretory reflex continues in long-term lactation for the first year post partum. In addition, breast stimulation in long-term lactating women continues to produce a slight increase in serum prolactin levels.