OBJECTIVES--To define relation between mood and concentrations of progesterone and cortisol during perinatal period to test hypothesis that rapid physiological withdrawal of steroid hormones after delivery is associated with depression. DESIGN--Prospective study of primiparous women from two weeks before expected date of delivery to 35 days postpartum. SETTING--Antenatal clinic in university hospital, obstetric inpatient unit, patients' homes. SUBJECTS--120 of 156 primiparous women interviewed. Remainder excluded because of major marital, socioeconomic, or medical problems or because caesarean section required. MAIN OUTCOME MEASURES--Concentrations of progesterone and cortisol in saliva samples; women's moods assessed by various scores for depression. RESULTS--Changes in salivary progesterone and cortisol concentrations were similar to those already characterised for plasma. Peak mean score for maternity blues (5.3 on Stein scale) was on day five postpartum (P < 0.02 compared with mean scores on other postpartum days). High postpartum scores for maternity blues were associated with high antenatal progesterone concentrations on day before delivery (P < 0.05), with high rate of rise of antenatal progesterone concentrations (P < 0.05), with decreasing progesterone concentrations from day of delivery to day of peak blues score (P > or = 0.01), and with low progesterone concentrations on day of peak blues score (P < 0.01). Seventy eight women were designated as having maternity blues (peak score > or = 8 on Stein scale) while 39 had no blues. Women with blues had significantly higher antenatal progesterone concentrations and lower postnatal concentrations than women without blues (geometric mean progesterone concentrations: one day before delivery 3860 pmol/l v 3210 pmol/l respectively, P = 0.03; ten days postpartum 88 pmol/l v 114 pmol/l, P = 0.048). Cortisol concentrations were not significantly associated with mood. CONCLUSION--Maternal mood in the days immediately after delivery is related to withdrawal of naturally occurring progesterone.