Plasma progesterone concentrations in rats were determined by a method employing gas-liquid chromatography with electron capture detection. Progesterone concentration was found to decrease before parturition and to increase again during lactation. A maximal post-partum concentration of this hormone was found during the 4th day of lactation. Removal of the litter or the placenta at delivery resulted in a decrease in the progesterone concentration at this time.

A diurnal variation of testosterone in the peripheral plasma of adult male subjects has been observed. Significantly greater quantities of the hormone (p<0.01) were found at time 0800 when compared to the levels in the afternoon or in the evening. Also, the plasma concentration of testosterone was higher on 2 consecutive mornings compared to the intervening evening. The magnitude of this diurnal variation was not so great as that reported for plasma PorterSilber chromogens or for conjugated dehydroepiandrosterone. (J Clin Endocr 26: 573,1966)

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