Administration of 25 mg Dinoprost (PGF 2α-THAM) to six cows on day 270 of gestation induced parturition in five animals within 48–72 h. Six other cows served as controls. Blood samples were collected from the jugular vein three times a day and the concentrations of epitestosterone, unconjugated oestrogens (ether extractable) and conjugated oestrogens (not ether extractable) were measured by radioimmunoassay. Epitestosterone and conjugated oestrogen levels were similar to those of the control group (5–7 and 11–24 nmol/l respectively). After parturition, the concentration decreased to basal levels. The concentration of unconjugated oestrogens reached maximum levels at birth but the absolute values were lower in the group with induced parturition (2.8 ± 0.7 nmol/l) compared to the control group (9.2±1.0 nmol/l).