With recent advances in molecular genetics, the popularity of mice as subjects for behavioral neuroscience is increasing at an exponential rate. Unfortunately, the existing body of knowledge on sexual behavior in male mice is not large and many basic gaps exist. The assumption that what is true of rats is also true of mice is a dangerous one that can misdirect and, in the worst case, impede progress. We summarize the current knowledge about the sexual behavior of male mice, with an emphasis on hormonal bases of these behaviors. Behavioral differences between strains, developmental actions of steroids, activational actions of steroids given peripherally and in the brain, and data generated in various receptor knockout and related mice are discussed. In addition, suggestions are made for the standardization of experimental protocols used in investigations of the sexual physiology and behavior of male mice in order to facilitate between-experiment and between-laboratory comparisons and to expedite the growth of knowledge in this area.