The ability of the flavonoid-rich fraction (5,7,3'-trihydroxy, 6,8,4'-trimethoxy flavones) of Vitex negundo seeds to antagonize the androgen action of exogenous testosterone propionate on the male reproductive system was studied in castrated prepubertal and intact adult dog. The flavonoid-rich fraction (F) was administered 10 mg/kg i.p. every other day either alone or in combination with testosterone propionate. The duration of treatment was 30 days in castrates and 60 days in intacts. Treatment with F caused disruption of the latter stages of spermatogenesis. The epididymides were devoid of spermatozoa. Protein, sialic acid and RNA contents of the testes and epididymides were reduced significantly while testicular cholesterol and phosphatase activity in testes and epididymides were elevated. Reduced androgen production was reflected in low levels of sialic acid in testes and epididymides. Castration alone brought about reduction in size and weight of the epididymides whereas castration followed by F treatment caused a reduction in the cellular heights of epididymides. Simultaneous administration of testosterone propionate to F-treated intact adult and castrated prepubertal dogs maintained the viability of spermatozoa, kept epididymal physiology relatively normal and significantly increased the cellular heights of epididymides.