The effect of suckling on serum cortisol concentrations was assessed in postpartum lactating women studied during serial breast feeding sessions 1-24 weeks postpartum. The mean +/- SD serum cortisol concentration at 15 min after the start of nursing, 9.8 +/- 3.89 micrograms/dl, was significantly lower, P = 0.001, than prior to the start of nursing, 13.2 +/- 5.92 micrograms/dl. The decline in the serum cortisol concentrations in the breast feeding women was not due entirely to the normal metabolism of the hormone or the normal circadian variation in cortisol secretion. These studies complement and expand upon a recent report  of a significant decrease in plasma adrenocorticotrophic hormone in breast feeding women studied one week postpartum. The neuroendocrine mechanisms responsible for this effect in women have yet to be defined.