Enlargement of seminal vesicles in C57BL/6J male mice occurs spontaneously in a subpopulation of mice aged 24 mo. or older. The enlargement is bilateral and results entirely from an increased mass of seminal fluid. Glandular hypoplasia is indicated by histological studies and DNA content. Thus, the enlargement of seminal vesicles differs from the phenomenon of benign hypertrophy of the prostate, which is commonly found in the corresponding age group of dogs and man.