Chorionic gonadotrophin (CG) is present in the placentae of man and rhesus monkey throughout pregnancy. In man the concentration of gonadotrophin is highest, and in the rhesus monkey it is lowest in early pregnancy. In man the concentration of placental CG falls as gestation proceeds; in the rhesus monkey there is a significant increase in monkey chorionic gonadotrophin (MCG) as the placenta ages. The marmoset term placenta has less gonadotrophin in it than either that of man or rhesus monkey.