ABSTRACT: The typical levels of inherent variability among untreated sexually mature rodents, rabbits, or men were determined for several endpoints used to characterize spermatogenesis. The latter included germ cell: germ cell and germ cell: Sertoli cell ratios, the frequency of specific stages of the cycle of the seminiferous epithelium, and the appearance of a variety of testicular abnormalities. Based on this variability, the number of replicates needed to provide future experiments of a predictable power and sensitivity was estimated. Replication requirements differed greatly as a function of species and/or among specific endpoints within a single species. In addition, the replication needed to provide robust experiments was substantially greater than that employed in most investigations. The application of these findings during the design and interpretation of future experiments was discussed.