When monospecific and well-characterized antiserum to highly purified ovine lh was administered to pregnant hamsters before Day 4, implantation of the ovum was inhibited. Injection of the antiserum between Days 5 and 11 resulted in termination of pregnancy. These effects were accompanied by a reduction in ovarian weights and luteal degeneration. A combination of progesterone and oestradiol-17 β were required for complete reversal of the effects of antiserum on implantation and pregnancy. The antiserum was not effective in terminating pregnancy when administered after Day 11. Injection of antiserum to ovine fsh did not alter the course of gestation. Of the three gonadotrophins (fsh, lh and prolactin) administered, only the fsh—lh combination could postpone parturition. From these observations, it can be concluded that deprivation of lh essentially affects luteal function in the pregnant hamster.