Injection of either testosterone enanthate or testosterone cypionate in equivalent doses yielded identical serum testosterone concentrations both interms of maximal concentrations and in terms of duration of elevation above basal levels. Maximal and supraphysiologic levels were achieved as early as the 1st day after injection, and these values had returned to basal concentrations on day 10. There were also no differences in the conversion of testosterone to DHT and in the suppression of LH and FSH observed. Thus, both esters show the same pharmacokinetic properties and appear to be equally useful for clinical purposes