In 29 apparently healthy subjects aged 19 to 23 years, with the same diet and physical exercise, urinary 17-ketosteroids (17-KS) were measured before and after a single intramuscular injection (in oil) of the equivalent of 200 mg of testosterone, administered in the form of the p-cyclopentyl propionate (CPP) to 9 subjects, of the enanthate (ENT) to 10 subjects, and of the p-hexoxyphenyl propionate (HPP) to 10 subjects. The average pretreatment 24-hour excretion in the 29 subjects was 10.6 mg of dehydroepiandrosterone equivalent with 95 per cent fiducial limits at 5.5 and 15.7 mg. From a complete analysis of variance of pretreatment excretion the 95 per cent fiducial limits of the "normal" day-to-day variation were assessed as 77 and 124 per cent. Administration of the different testosterone esters resulted in an increased 17-KS excretion of varying degree and duration; the effect of CPP and ENT lasted at least five but less than eight days, whereas that of HPP lasted at least eight but less than eleven days. When compared with the pretreatment levels, the period of increased 17-KS excretion was followed by a transitory but significant reduction in the CPP-treated group, by a probable but not certain reduction in the ENT-treated group, and by unchanged urinary 17-KS levels in the HPP-treated group. The excretion of 17-KS was significantly greater following the administration of CPP and ENT than following the injection of HPP; the average "conversion rate" ([plus or minus] standard deviation of a single estimate) calculated from the excretion data of 7 out of the first 9 post-injection days was 20.1 [plus or minus] 11.0 per cent for CPP, 16.9 [plus or minus] 8.1 per cent for ENT, and 4.8 [plus or minus] 5.9 per cent for HPP. The large individual variations in the "conversion rate" of esterified testosterone suggest similarly large variations in other steroid conversion reactions in the human. Therefore caution is needed in the interpretation of quantitative data obtained in steroid metabolic studies carried out on a limited number of subjects.