An influence of thyrotropin (TSH), dibutyryl cyclic AMP (dbcAMP), triiodothyronine (T3) and thyroxine (T4) on the mitotic incidence in the organ-cultured rat thyroid was studied. Biphasic inhibitory and stimulatory effects of TSH on the thyroid mitotic activity were revealed. The inhibitory effect occurred 14 h after an exposure of the thyroid explants to TSH and was enhanced by T3. A similar inhibitory effect was observed after an exposure of the explants to dbcAMP. Twenty-four hours after the exposure of the thyroid explants to TSH, a stimulatory effect on the mitotic incidence was observed. The latter effect was not reproduced by dbcAMP.