SUMMARY Plasma progesterone concentration was measured by a competitive protein-binding method in serial samples of jugular venous blood from 21 pregnant goats, 11 with twin and 10 with single foetuses. Progesterone levels in twin pregnancies were significantly greater than in singletons. The mean progesterone concentration (ng/ml plasma) in the twin pregnancies was greatest during the 3rd month of gestation (10·7 ± 0·4 (s.e.m.)) and in the singletons during the 4th month (7·8 ± 0·2 (s.e.m.)). There was a significant decrease in mean progesterone concentration in the last month of pregnancy due to a steady decline in the last 7 days before parturition. The mean progesterone concentrations at parturition in five twin and eight single pregnancies were 2·2 ± 0·4 and 1·5 ± 0·2 (s.e.m.) ng/ml plasma respectively; there was no significant difference between these values. In cord blood from nine kids immediately after delivery the progesterone concentration was 0·9 ± 0·1 ng/ml.