A cytometric study was made of the effects of castration and hormone replacement on the secretory epithelium of seminal vesicles of sexually mature rats. Average diameters of equivalent circular cross sections of nuclei were calculated from nuclear areas obtained by camera lucida drawings. Cell heights were measured with an ocular micrometer. Twenty-four hours following gonadectomy the average nuclear diameter showed an insignificant decrease of 0.1 [mu] whereas cell height was reduced by 4 [mu] (P=0.01). The diameter fell 0.5 [mu] (P=0.10) and the height 8 [mu] (P=0.01) in 60 hours after castration. A total reduction in nuclear diameter of 1 [mu] (P=0.01), and in cell height of 11 [mu] (P= 0.01), was noted during the 38-day period of observation. In hormone-treated animals (500 Mg testosterone propionate in oil daily) the nuclear diameter increased 0.7 [mu]. (P=0.01) and cell height 1 [mu] (P=0.10) in the 12 hours following the initial administration of the hormone. By 60 hours the diameter had increased 2 [mu] (P=0.01) and the height 9 [mu] (P=0.01) above the value of the 38-day castrate. After this time the nuclear diameter showed a decrease whereas the cell height remained relatively constant at a value comparable to that of the intact animal. Sixty hours after gonadectomy most of the nucleoli were small and some nuclei were pyknotic. In the 38-day castrate general pyknosis was apparent. Forty-eight hours after initial hormone treatment the nucleoli were large and the nuclei had lost their pyknotic characteristics. No mitoses were observed in the secretory epithelium of castrates, or the 24- and36-hour hormone-treated animals. A mitotic activity of 7.5% was noted after 60 hours. Following this maximum there was a sharp decline in mistoses and in animals treated with male hormone for 20 days a value of 0.1% was observed.