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5alpha-Androstenone2) has been measured in pig peripheral plasma by radioimmunoassay (RIA). In 73 mature boars values ranging from 1.2 ng to 54.1 ng per ml plasma with a mean value of 18.3 ng/ml and SD equal to 15.9 were found. In female pigs and castrated male pigs the mean concentrations were 2.3 ng/ml (n equal to 18) and 1.1 ng/ml (n equal to 19) with SD equal to 0.8 and SD equal to 0.3 respectively. Clear-cut diurnal variations in the concentration of 5alpha-androstenone in plasma samples from boars were not observed. Intravenous injection of HCG in boars caused an abrupt rise in the 5alpha-androstenone level reaching a maximum in 90 min. Twenty-eight hours after the injection a second peak of plasma 5alpha-androstenone was observed. Following castration of boars the level of 5alpha-androstenone in peripheral plasma fell within 4 days, to levels found in female pigs.