[en] Sprague-Dawley rats were irradiated continuously (⁶⁰Co, γ) at a dose-rate of either 1 (0.0007 rad/min), 3 or 6 rad/23-h day for monthly intervals of 1 to 6. At one month after irradiation, counts of differentiating spermatogonia (A₁ and A₄) were reduced to a level that remained essentially unchanged during the succeeding 5 months of irradiation. The magnitude of the irradiation effect was greatest at stage 1 of the spermatogenic cycle where numbers of differentiating spermatogonia were reduced to 50% of control by 1 rad/day and to 30 and 20% of control by 3 and 6 rad/day, respectively. Number of stem spermatogonia was not significantly affected by 1 rad/day (P > 0.10). At 3 and 6 rad, however, number of stem cells declined from 80% of control at 1 month to 60 and 40% of control at 6 months, respectively. At a dose-rate of 1 rad/day there was neither a reduction in number of A₁ spermatogonia per clone nor significant evidence of necrosis among either stem or differentiating spermatogonia; therefore, a reduction of stem-cell mitotic activity appears to be the principal effect of continuous low-level irradiation on spermatogenesis in the rat. (Auth.)