. The concentrations of total and free progesterone, oestradiol-17β, and cortisol were measured in the plasma of 24 pregnant women from 10–17 days prepartum, during labour. and for 2–3 days postpartum. All the women were delivered vaginally, and their hormonal profiles were similar, whether they initiated labour spontaneously (n=11) or were induced by rupture of the fetal membranes (n=13). Before labour, the plasma concentrations of total radioimmunoassayable progesterone, oestradiol-17β and cortisol were 557 3 (SEM 70.3) nmol/l and 590 0 (SEM 76.7) nmol/l, 94 3 (SEM 18.7) nmol/l and 111 6 (SEM 19.8) nmol/l, and 1087.2 (SEM 103.7) nmol/l and 1020 7 (SEM 100.7) nmol/l, for the spontaneous and induced groups, respectively. During labour progesterone and oestradiol-17β declined in maternal plasma to around 130 nmol/l and 30 nmol/l in both groups by the end of the third stage. The free fractions of progesterone and oestradiol- 17β remained constant throughout labour and over the entire study period at 2–3% and 1–2%, respectively. Total plasma cortisol rose to 1243.7 (SEM 83.3) nmol/l (P<0.02, spontaneous group) and 1247.5 (SEM 97.4) nmol/l (P<0 001, induced group) by the end of the first stage of labour. Concomitantly, free cortisol increased from 4.5–5.5% to 7.2–8 2% over this period so that its concentration increased by approximately 2 5-fold in both groups of women during labour. We conclude that the total and physiologically-active free concentrations of progesterone and oestradiol-17β in maternal plasma do not change immediately before the onset of labour. That is, the onset of human parturition does not involve ‘progesterone withdrawal’ or ‘oestrogen augmentation’ in peripheral plasma. Total and free concentrations of cortisol are elevated in maternal plasma during labour possibly due to maternal stress.