Several spermatogenic cell types have been reported to be responsive to testosterone (T) in vivo. We have proposed that the principal action of T is to facilitate the maturation of round to elongated spermatids during spermiogenesis. To identify T-dependent cell types during spermiogenesis, round spermatid populations were counted using stereological techniques in adult rats after T withdrawal and replacement. The number of round spermatids per testis in stages I-III, IV-VI, VII, and VIII of the spermatogenic cycle were determined and, based on the known duration of each stage, the hourly production rates of round spermatid populations calculated. Sprague-Dawley rats received 3 cm T and 0.4 cm estradiol implants (TE treatment) for 11 weeks to suppress LH, testicular T levels, and spermatogenesis. To restore sperm production, high-dose T (24 cm) implants were then given, and animals were perfused 0, 2, 4, and 7 days later. Total testicular T levels were suppressed to 2.9% of control levels by TE treatment and significantly increased (P < 0.05) after 2 days of high-dose T, to remain between 7-12% of control. The hourly production rates of round spermatids between stages I and VII were suppressed to 29-35% of control by TE treatment and remained unchanged by high-dose T administration. Stage VIII round spermatid hourly production rates however, were markedly reduced to 5% of control by TE treatment and increased significantly (P < 0.05) to 27% of control after 4 days of high-dose T. The efficiency of conversion of spermatids through spermiogenesis was estimated from the ratios of the hourly production rates of successive spermatid groups. The conversion of spermatids between stages I-VII of the cycle did not differ from control regardless of the treatment. However, the conversion of round spermatids between stages VII and VIII was markedly suppressed to 16% of control by TE treatment and was then normalized after 4 days of high-dose T administration. We conclude that the conversion of round spermatids between stages VII and VIII is a highly T-dependent step during spermiogenesis.