Alterations in plasma prolactin (PRL) concentrations in response to nursing in puerperally lactating women are often significant beyond the 90th postpartum day, yet the increment appears unrelated to the frequency or duration of the nursing stimulus. Tonic gonadotropin secretion is low, assuming a more episodic secretory pattern either when the frequency of breast-feeding is reduced or when weaning takes place. Significant increments in peripheral concentrations of luteinizing hormone can be seen in response to weaning coincident with a fall in peripheral plasma PRL concentration. At the same time, peripheral estrogen concentrations increase, suggesting that a specific set point for ovarian responsiveness to gonadotropins exists. Whether this set point is related solely to the peripheral concentration of gonadotropins or whether it is also related to the peripheral PRL concentration is not known at this time.