SUMMARYLevels of progesterone, oestradiol and aldosterone in the plasma of twenty women with premenstrual syndrome (PMS) were measured by radioimmunoassay and the results were compared with those from a control series of ten normal volunteers. All the PMS patients were apparently ovulatory but the pooled progesterone values from days −9 to −5 premenstrually were significantly lower than the control values (P<0·05). The oestradiol levels in the PMS patients were higher than in control subjects over the last 4 days of the cycle (P<0·01). There was no significance in aldosterone levels between the PMS and control groups. The relevance of these results to the aetiology of PMS is consi