Serum concentrations of progesterone and prolactin, and luteal 3betaHSD and 20alphaHSD activities in rats throughout pseudopregnancy Pseudopregnant rats were sacrificed at 18:00 h on each of the days of PSP studied. Trunk blood was obtained after decapitation and serum progesterone (Panel A) and PRL (Panel B) concentrations measured by RIA. The CL were isolated and 3betaHSD (Panel C) and 20alphaHSD (Panel D) activities were measured spectrophotometrically. Results are mean ± SEM of groups of 5–12 animals. Columns with different letters differ significantly. Panel A: a vs. b, p < 0.05. Panel B: a vs. b, p < 0.001. Panel C: a vs. b, b vs. c, p < 0.01; a vs. c p < 0.001. Panel D: a vs. b, p < 0.001.