1. Intravenous injection of [3H]progesterone in non-pregnant monkeys resulted in total disappearance of the labelled hormone from the circulation within 3 h. However, 0.5-1.75 h after disappearance the hormone reappeared, reaching 20% (median, 5%) of the initial maximal concentration. 2. Reappearance of labelled hormone was accompanied by similar fluctuations in the levels of labelled metabolites, [3H]20α-dihydroprogesterone and [3H]17α-hydroxyprogesterone which reached 61% (median, 14%) and 120% (median, 13%), respectively, of the initial maximal concentrations. 3. Chromatography was used to separate labelled progesterone and its metabolites. Efficiency of the procedure was determined separately in each sample and for each steroid. All data were corrected for percentage recovery. 4. Analytical equations were devised, based on the theory of compartmental systems with continuously distributed time lags, to describe the unexpected kinetics of progesterone levels. The coefficients of determination ranged from 86 to 99% (median, 96%) which indicates that the equations enabled reliable prediction of hormone levels in blood within the time range studied. 5. The unexpected reappearance of labelled progesterone cannot be explained by hormone secretion but only by a delayed release from tissue stores, since progesterone does not undergo enterohepatic recirculation. Thus, a previously undescribed mechanism affecting circulating progesterone levels, and perhaps those of other hormones, exists.
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