The effect of oestradiol benzoate on serum gonadotrophin concentrations before and after LHRH administration was studied in lactating and non-lactating women at 3 and 6 weeks post-partum. Except in the non-lactating women at 6 weeks, basal serum FSH concentrations were suppressed by oestrogen. There were no significant changes in basal concentrations of LH after oestrogen in the lactating women in either the 3- or 6-week studies. Individual increases in the basal LH concentrations in two out of six non-lactating subjects in the 6-week study occurred but overall there were no significant changes. In the 6-week study amplification of the LH response to LHRH was found in both groups, the effect being significantly greater in the non-lactating women. Overall FSH responses were also significantly different in the two groups, being suppression in those lactating ad amplification in those not lactating. The LH/FSH ratios following LHRH administration in the 6-week non-lactating study were similar to those seen in the early follicular phase in regularly menstruating subjects. The basal ratios in the lactating subjects were, however, significantly less than those seen in the non-lactating subjects both at 3 and 6 weeks. This difference was associated with the relative enhancement of LH release in non-lactating subjects and enhancement of FSH release in those lactating. Taken together the results indicate the presence of an intact negative feedback of oestrogen on gonadotrophin release in both groups being enhanced at 6 weeks post-partum in the lactating subjects; also in the lactating subjects at 6 weeks there was less amplification by oestrogen of the responsiveness of the anterior pituitary to LHRH. At 6 weeks, however, in the non-lactating group these responses were similar to those seen in normal regularly menstruating subjects. These dynamic endocrine studies suggest a possible hypothalamic-pituitary mechanism which may help to explain the delayed return of ovulatory cycles in lactating women.