The concentrations of androgen, progesterone and prolactin-like activity in serum, placentae and media from placental incubations during the second half of pregnancy in the C3H mouse were evaluated. Serum concentrations, placental content and in-vitro placental release of androgen were raised on day 10 of pregnancy. Serum progesterone levels showed minor fluctuations during the second half of gestation, whereas placenta content and in-vitro release of progesterone were increased on day 10 of gestation. The serum profile of prolactin-like activity showed a significant mid-pregnancy increase on day 10 which did not correlate with placental content or in-vitro placental release of prolactin-like activity. The placental content and in-vitro release of prolactin-like activity were low during mid-pregnancy and increased during the latter days of gestation when serum prolactin-like activity was reduced.