The effect of continuous light on luteal function was studied in both adult pseudopregnant (induced by cervical stimulation) rats and in immature rats with “superluteinized” ovaries (50 IU PMS, 25 IU hCG). All rats were maintained in a photoperiod of 14 hr light and 10 hr dark (CL) prior to induction of pseudopregnancy or gonadotropin treatment. Exposure to continuous light (LL) was started at the time of cervical stimulation and PMS administration for adult pseudopregnant and immature rats, respectively. Adult rats were killed on days 3, 5, 7, 9, 11 and 12 of pseudopregnancy. PMS—hCG treated (PMS—hCG—Tr) animals were sacrificed on days 3, 5, 7, 9, 12, 14, 17 and 21 following hCG administration. Progesterone and 20a—OH—P were extracted from ovarian venous blood of adult pseudopregnant rats and from systemic blood of immature PMS—hCG—Tr animals. Extracts were purified through thin—layer chromatography (TLC). 20a— OH—P was acetylated and rechromatographed. Both steroids were quantified by gas—liquid chromat...