ABSTRACT: Sperm production is an important variable affecting the reproductive capacity of men and other male mammals. Because spermatogenesis is highly susceptible to disruption, volume density techniques that enable the composition of testicular tissue to be characterized or sperm production rates to be quantified are used extensively to assess potential impacts of known or suspected reproductive toxins, the safety of proposed human or animal drugs, and basic studies on spermatogenesis in normal individuals. The number of subjects used per treatment group for such studies has been variable. However, the power and sensitivity of any experiment is dependent on the inherent variability associated with the end point(s) of interest and the number of replicates (ie, animals or men per treatment group) employed per treatment group. Because the reliability of one's experimental outcome should be of utmost consideration, it is important to characterize the typical levels of inherent variability associated with one's chosen end point(s) and to answer the question: how many subjects are required per treatment group to provide an experiment with a given power and sensitivity for detecting actual treatment effects? This study was undertaken to 1) characterize the inherent variability associated with the volume density of several testicular components in rodents, rabbits, and humans and among cell numbers derived from volume density data and 2) identify the approximate number of replicates that would be required to provide future studies of predictable power and sensitivity for which data were to be generated via the volume density approach. Replication requirements differed, sometimes by several orders of magnitude, among these species and among end points within a single species. In addition, for many of these species and end points, the number of replicates necessary to ensure modest power and sensitivity for detecting treatment differences exceeded that used in most investigations. These findings are discussed in relation to the design and interpretation of future investigations with these species and end points.