Azadirachta indica (Neem) seed oil brings about a significant diminution in spermatozoan motility and density. It leads to reduction of fertility rate in rats and rabbits. The body weight of the animals remain unaffected but the weights of reproductive oranges declined. Reduction of cauda epididymal protein, sialic acid, acid phosphatase and seminal vesicular fructose concentration in rats and rabbits could bedue to antiandrogenic action of the seed oil as confirmed with the help of bioassay techniques. Histopathology of testis revealed arrest of spermatogenesis and sever degenerative changes in the cauda epididymis.