SYNOPSIS Thirty-four women who had vaginal deliveries of live infants completed behavioural ratings and supplied blood and 24-hour urine samples on three occasions during pregnancy and on three occasions after delivery. Approximately one third of the women indicated a distinct upswing in mood between Days 2 and 4 following delivery. These 'positive mood change' subjects showed changes in urinary cyclic AMP (adenosine 3'5' cyclic monophosphate), plasma cyclic AMP, whole blood cell cyclic AMP, whole blood cell ATP (adenosine triphosphate), haematocrit and urinary 11 OHCS (11-hydroxycortisol steroids) following delivery which were different from those observed in the rest of the subjects and comparable with the biochemical changes described during upswings in mood in short-cycle manic-depressive subjects. There was also an indication that the women showing this upswing in mood following delivery were distinct in some respects on both behavioural ratings and biochemical findings during pregnancy.