This study was designed to examine the state of proliferation in the rat thyrocyte following the administration of thyroid stimulating hormone (TSH). An immunohistochemical technique involving the use of a monoclonal antibody to statin, a nonproliferation-specific nuclear antigen, was developed to measure the subpopulation of cells that have ceased to divide. Following the random assignment of young male Sprague-Dawley rats into various groups, the rats in the control group received a single intraperitoneal (i-p) injection of normal saline, whereas the experimental groups received single i-p injections of TSH at doses of 0.25, 0.50, and 1.0 IU, respectively. All rats were subsequently sacrificed in groups of three at 1, 2, 4, and 24 hours. The statin antibody label was readily identified within the follicle cell nucleus. Results revealed a statistically significant transient decrease in the mean percent statin-positive nuclei in the TSH-treated groups. The time- and dose-dependent effect of TSH was maximal at 2 hours and no longer discernible at 24 hours. A second experiment involving the chronic administration of TSH (i-p 0.25 IU twice daily) resulted in a cumulative response with a statistically significant progressive decrease in the mean percent of statin-positive nuclei at 5 and 10 days, returning to near normal values 5 days following the cessation of treatment. Determination of the nuclear optical density of the statin reaction product by image analysis techniques revealed that a single injection of TSH resulted in a rapid disappearance of the statin nuclear protein. This result suggests that the disappearance of statin in the nucleus appears to reflect the event of cells leaving the nondividing quiescent state to resume the cell cycle traverse following the administration of TSH. The disappearance of statin appears as an early nuclear event that parallels the earliest known cytoplasmic pinocytotic response to TSH in the rat thyroid follicle cell.