Sperm capacitation (1,2) in the rabbit uterus occurs more rapidly when estrogen is the predominant gonadal hormone (3). Soupart (4, 5) reported that sperm capacitation proceeds more rapidly after mating than after surgical insemination into the uterine lumen, and that an injection of 75 IU of HCG enhances sperm capacitation similarly to mating in rabbits. These observations prompted us to compare the influence of HCG with that of LH on sperm capacitation.Materials and Methods. Mature estrous rabbits of mixed breeding and weighing 3 to 5 kg were used for capacitator (6) and test rabbits. Each rabbit was caged individually for at least 1 month a t 18' with a 14-hr photoperiod before assignment to an experimen t.In any experiment, semen samples were pooled from 2 to 4 rabbits with proven fertility. Seminal plasma was discarded and the sperm were resuspended to 500 million/ml of Locke's solution with 100 pg of dihydrostreptomycin sulfate /ml. Under ether anesthesia, the uterine horns of capacitator rabbits were exposed via a midventral incision and 90 to 200 million sperm (at least 50% motile) were injected within each uterine horn near the tubouterine junction with a 26-gauge needle.