Testosterone and 5alpha-dihydrotestosterone inhibit both growth and spermatogenesis in the remaining testis in partially castrated adult newts, Notophthalmus viridescens.

The testis of the newt, Notophthalmus viridescens, undergoes hypertrophy following partial castration. Injections of either testosterone or 5alpha-dihydrotestosterone are equally effective in inhibiting the increase in testis weight which follows partial castration. In addition, both of these androgens inhibit spermatogenesis, i.e., spermatogonia appear to be inhibited resulting in early cessation of spermatogenesis and a precocious division of the testis into separate spermatogonia and sperm compartments.