In the neurological mutant mouseweaver(wv/wv), the majority of males are infertile due to hypospermatogenesis. Heterozygous weaver mice (wv/+) cease mating successfully when males reach an average age of 3.5 months. The contents of epididymal fluid were scored for the number of sperm and sperm motility inwv/wv,wv/+ and controls. Testes were examined in mice of the three genotypes at various ages using light and electron microscopy. Inwv/+ males, sperm counts were significantly lower than in controls and were significantly higher than inwv/wv.The seminiferous epithelium in weaver mice appears depleted soon after puberty and a wide range of degenerative changes was identified in both germ cells and Sertoli cells. Analogous cellular aberrations were detected in heterozygous males, but they appeared at an older age and were not as severe as inwv/wv.We hypothesize that in weaver homo- and heterozygosity the damage of Sertoli cells may induce degeneration of germinal cells and particularly affect the most advanced spermatogenic cells.