SUMMARY The concentrations of ovine placental lactogen (oPL) have been determined in maternal plasma, umbilical cord plasma, and allantoic fluid by an homologous radioimmunoassay for oPL which is sensitive to 0·1 ng hormone. Ovine placental lactogen was first detected in maternal plasma at 41–50 days of gestation and reached a peak concentration of 2547 ± 226 (s.e.m.) ng/ml at 121–130 days in ewes with singleton gestations. The oPL concentration in cord plasma was 336·4 ± 60·3 ng/ml and in allantoic fluid was 29·6 ± 6·4 ng/ml. After surgical removal of the placenta, oPL disappeared from maternal plasma with a half-life of 29·1 ± 1·3 min.