Endröczi & Lissák (1962) and Saginor & Horton (1968) have shown that in rabbits the testosterone output in venous testicular blood increases shortly before and after copulation. Drori & Folman (1964) and Folman & Drori (1966) have shown that in male rats mating produces hypertrophy of the accessory reproductive organs and suggested that this is due to increased integral androgen production. Drori, Amir & Folman (1968) have also shown increased fructose levels in the coagulating glands of mated male rats. In this study, the testosterone content in the testes of mated and unmated male rats was compared. Forty-eight pairs of litter-mate rats aged 40 days were split into two equal groups: in one group the rats were mated and in the other they were unmated. The mated male rats were housed with hysterotomized females in groups of four males to two females; the females were replaced once a week. The