One of the characteristics, which is perhaps most remarkable and in some ways unique, is the specific dependence of the function of organs such as the prostate and seminal vesicle upon the presence of androgenic stimuli. In its extreme form this dependence is clearly reflected in the well documented and much studied set of phenomena of post-castrate involution and testosterone-induced resump- tion of growth in the accessory organs. Testosterone acts on the male accessory organs directly, without having first to undergo metabolic alteration in the animal body. Convincing proof of such direct action was provided by an experi- ment recently carried out on bull seminal vesicles in vivo, in which testosterone was injected locally into one seminal vesicle in situ, while the other was left as an untreated control: 3 weeks after local introduction of 10 mg testosterone oenanthate, the treated seminal vesicle was twice as large and contained four times as much fructose and citric acid as the untreated control (Mann, Rowson, Baronos & Karagiannidis, 1971). In the course of the same investigation we have also compared the stimulating potency of testosterone with that of 5a-dihydrotestosterone, an androgenic steroid to which testosterone is known to be converted in the male accessory organs (Bruchovsky & Wilson, 1968; Anderson & Liao, 1968; Wilson & Gloyna, 1970; Williams-Ashman & Reddi, 1971). We found that, when introduced directly into the seminal vesicle, dihydrotestosterone oenanthate was capable of promoting growth and formation of fructose and citric acid, but the magnitude of the effect was no more than that of a corresponding dose of testosterone oenan¬ thate. As a matter of fact, in another experiment, in which two castrated twin bulls were injected subcutaneously, one with dihydrotestosterone and the other with testosterone, the latter responded with significantly greater secretory output of fructose and citric acid in electro-ejaculated seminal plasma. However, the growth of the seminal vesicles in both twins was about the same.