The pathologic phenotype of the testis in both prepuberal and postpuberal male weaver mutant mice was studied by light microscopy. Morphometric analysis of seminiferous tubules was carried out. Epididymal fluid was examined for the presence of spermatozoa. The seminiferous tubules of 21-day-old prepuberal weaver mutant mice lacked patent lumina and had more degenerated cells than control mice. Fifty-six day-old weaver mutants had many germinal epithelial cells located within the adluminal compartment that were in advanced stages of degeneration. Round spermatids were enlarged and multinucleated. Round spermatids and spermatocytes had sloughed into the lumen. Compared to control mice, elongated spermatids were seen less frequently. In older weaver mice, the degenerative process involved germ cells in both the adluminal and basal compartments. In 143- and 226- day-old weaver mutants, the Sertoli cells were atrophic. Diameters of seminiferous tubules in weaver mice were significantly reduced when compared to control mice. Sperm were either absent or very low in number in the epididymal fluid of postpuberal weaver mice. We conclude that spermatogenesis is abnormal in male weaver mutant mice. The testicular phenotype is characterized by a degenerative process that affects both germ cells and supporting cells.