To elucidate the role of prolactin (Prl) in the endocrinology of early human pregnancy, 28 healthy women were given 5.0-7.5 mg of bromocriptine daily for two weeks between weeks 6-9 of normal gestation. Plasma Prl, oestradiol-17 beta (Oe2), progesterone (P), testosterone (T) and human placental lactogen (hPL) were measured before, and one and two weeks after the start of bromocriptine, and they were compared with those in 22 control women who were followed similarly but without bromocriptine treatment. Bromocriptine treatment induced a significant Prl depression at one week (7.3 +/- 4.3 ng/ml vs 23.7 +/- 11.4 ng/ml) and two weeks (5.3 +/- 2.5 ng/ml vs 31.9 +/- 16.4 ng/ml). Oe2, P, T and hPL levels, however, showed no significant differences between the groups. Two women undergoing bromocriptine treatment (7.1%) and one control women (4.5%) experienced spontaneous incomplete abortion during the study period, but these three already had a low Oe2 level and a low/or undetectable hPL level at the beginning of the study. It is obvious that neither maternal 'hypoprolactinaemia' nor bromocriptine during early human pregnancy interfere with the normal progress of pregnancy or with the normal synthesis of sex steroids and hPL at this time.