Testosterone-filled polydimethylsiloxane (PDS) capsules, ranging between 800 and 25,600 mm2 in surface area, were placed subcutaneously in mature male rabbits to examine the capacity of testosterone to 1) maintain the seminiferous epithelium, and 2) reinitiate spermatogenesis after suppressing germ cell development with testosterone. Placement of 800 mm2 PDS testosterone capsules for 60 or 65 days consistently induced gonadal atrophy whereas capsules providing 25,600 mm2 maintained normal testis weight and spermatogenic activity. Importantly, serum LH declined at least 7-fold below normal in rabbits receiving PDS implants inducing testis atrophy or supporting spermatogenesis. In contrast to LH, plasma testosterone levels increased in proportion to the surface area of PDS capsules between 3,200 and 25,600 mm2. Increases in seminal vesicle weight paralleled increases in plasma testosterone titers but neither body weight nor prostate, vesicular and bulbourethral gland weights were affected (p > 0.10). Complete restoration of spermatogenesis and fertility occurred within 75 days after removing 800 mm2 PDS capsules. However, only partial restoration of spermatogenesis followed placement of 25,600 mm2 capsules capable of maintaining spermatogenesis once gonadal atrophy was induced with 800 mm2 PDS capsules. It was concluded that this mode of testosterone administration quantitatively maintained spermatogenesis when plasma testosterone concentrations ranged above 40 ng/ml and that similar levels of testosterone reinitiated spermatogenesis to 80% of control values once testis atrophy was induced with 800 mm2 PDS testosterone capsules. (Endocrinology93: 450, 1973)