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Summary. The supernatant resulting from centrifugation of a homo- genate of the central portion of the guinea-pig placenta at 1000 g was found to synthesize progesterone from endogenous precursor. Addition of an NAD- or NADPH-generating system stimulated progesterone production. Placental steroidogenesis in the guinea-pig is of special interest because this animal shares with the human the ability to maintain pregnancy if ovariecto- mized during the latter half of gestation. This observation indicates that the guinea-pig placenta synthesizes progesterone. Heap & Deanesly (1966) found that the plasma progesterone concentration was correlated with placental weight in the ovariectomized guinea-pig. They also reported a fivefold higher progesterone level in the plasma after passage through the uterus. However, Ferguson & Christie (1967), using histochemical methods, were unable to detect -3/í-hydroxysteroid dehydrogenase in the guinea-pig placenta during the latter portion of gestation. Recently, Bedwani & Marley (1971) were able to show conversion of exogenous pregnenolone to progesterone by guinea-pig placental slices. This report describes the biosynthesis of progesterone in the guinea-pig placenta from endogenous steroid precursor. Placentae were obtained by Caesarean section from randomly bred guinea- pigs during the last 5 days of pregnancy as determined by the width of the pubic opening (Phillips, Wolfe & Gordon, 1959). The cord and the decidua basalis were discarded. The central placental tissue located between the cord and the mesometrial pole of the uterus were dissected free and homogenized in a hand homogenizer with a solution containing 0-25 M-sucrose: Krebs' Ringer phos¬ phate buffer, pH 7-4: 0-04 M-nicotinamide (11:5: 1), using 1 ml of solution for

BIOSYNTHESIS OF PROGESTERONE FROM ENDOGENOUS STEROID PRECURSOR BY THE GUINEA-PIG PLACENTA