The goal of this morphometric study was to obtain quantitative information on the seminiferous tubules of Sprague-Dawley rats, including changes seen at various stages of the cycle of the seminiferous epithelium. Tissue from perfusion-fixed testes was embedded in Epon-Araldite; and sections were subjected to morphometric measurements at the light microscopic level, using point counting for volume densities and the Floderus equation for numerical densities. Changes occur in the diameter of the seminiferous tubule, as well as in the volume of the seminiferous epithelium and tubule lumen, from stage to stage during the cycle. A significant constriction of the seminiferous tubule accompanies spermiation. The volume of the seminiferous epithelium per unit length of the tubule begins to increase after stage XIV, and peaks at stage V of the next cycle. The tubule lumen increases dramatically from stages V to VII, at the expense of the epithelium. The number of Sertoli cells is constant per unit length of the seminiferous tubule at all stages of the cycle. This is also true for primary spermatocytes of various developmental phases and for round spermatids from step 1 through step 10 of spermiogenesis. The average number of younger (preleptotene, leptotene, zytgotene) primary spermatocytes per Sertoli cell is 2.34 +/- 0.082 (SEM), the number of older (pachytene, diplotene) primary spermatocytes per Sertoli cell is 2.37 +/- 0.064, and the ratio of step 1-10 spermatids to Sertoli cells is 7.89 +/- 0.27. By studying tangential views of serially sectioned seminiferous tubules at stage V, it is shown that the number of step-17 spermatids associated with each Sertoli cell averages 8.35 +/- 0.128, although the counts ranged from 6 to 11. The only appreciable occurrence of cell death after the last spermatogonial mitosis appears to be a 15% loss during the first meiotic division. From our morphometric results, corrected for volume changes during preparation for microscopy, there are 15.7 million (+/- 0.99 million) Sertoli cells per gram of fresh rat testis. The length of seminiferous tubule per gram of testis is estimated to be 12.4 +/- 0.56 meters, and the tubule surface area per gram testis is 119.7 +/- 2.57 cm2. The daily production of mature spermatids is 9.61 million (+/- 0.615 million) per gram of testis.