Quantitative (stereological) studies were performed to determine the number of germ cells in the developing rat testis. Sprague-Dawley rats aged 1-70 days were fixed by immersion or perfusion and embedded in Epon Araldite. Blocks of tissue were sectioned at 1.5 microns and stained with toluidine blue dye. Sections were systematically scanned and the areal density of nuclear profiles counted using an unbiased counting frame. Numerical density and absolute number of germ cells in the processed block were then estimated. Corrections for processing shrinkage were determined by comparing the volume of processed and unprocessed samples. The results demonstrate the necessity of determining absolute number rather than volume density (or areal density) in comparing germ cell numbers. In these experiments, spermatogonial numbers stabilized in the range 18.4-23.6 million per testis on Day 30. The number of primary spermatocytes that were first apparent on Day 15 increased rapidly to 54.6 million per testis on Day 30 and then slowly to 73.6 million on Day 70. Round spermatids were first apparent on Day 25 and increased rapidly to 85.7 million per testis on Day 40, then continued to increase to 151.9 million on Day 70. The study provides both methods and baseline data for future experiments involving manipulation of the spermatogenic potential of the testis.