OBJECTIVE The objective of this study was to investigate serial changes in the plasma concentration of inhibin in both the very early days of pregnancy following implantation and in late pregnancy. The timing of the changes in inhibin concentration relative to changes in the concentrations of other hormones of pregnancy was also investigated. METHODS Serial observations of the peripheral concentrations of inhibin and other hormones in two groups of healthy volunteers in (a) early pregnancy and (b) late pregnancy. METHODS (a) Four healthy women recruited on cessation of contraception prior to conception. (b) Nine healthy women recruited at the antenatal clinic. METHODS In the early pregnancy subjects, the concentrations of inhibin, progesterone, oestradiol and hCG were measured in plasma samples obtained three times per week from day 8 to day 10 of each menstrual cycle until 11 weeks after the last menstrual period in the conception cycle. In the late pregnancy subjects, plasma samples were obtained at 4-week intervals from 12 weeks until term. RESULTS The concentration of inhibin, progesterone and oestradiol in conception cycles were similar to those in the preceding cycles until the mid/to late-luteal phase of the cycle when hCG was first measureable. By day 12 of the luteal phase the concentration of inhibin was significantly higher in the pregnancy cycle than in the non-pregnancy cycle (P < 0.05) and progressively increased after the time of the missed menstrual period. The concentration of inhibin reached a peak (513.0 U/l, CI 442.1-595.3) by day 47 when the concentration of hCG was maximal. In early pregnancy the concentration of inhibin was correlated with that of hCG (r = 0.361; P < 0.01) as well as progesterone (r = 0.584, P < 0.001) and oestradiol (r = 0.602, P < 0.001). After 12 weeks there was no significant correlation between hCG and inhibin although significant correlations persisted with progesterone (r = 0.553, P < 0.001) and oestradiol (r = 0.361, P < 0.01). CONCLUSIONS The corpus luteum makes a significant contribution to the production of inhibin in early pregnancy while after 12 weeks the placenta is the major source.