In the female rabbit, a reflex ovulator, serum LH and FSH levels after mating were measured by radioimmunoassay. The ovaries were examined 48 hr later for ruptured ovarian follicles. Coitus resulted in ovulation after some hours and was followed by a rapid increase in serum LH levels; no change in LH serum levels was observed when coitus was not followed by ovulation; conversely, no significant changes were observed in serum FSH levels after coitus regardless of whether or not ovulation occurred. The major finding of the present study is that, in the rabbit, the ovulatory surge of gonadotropin includes LH but not FSH. It is suggested that LH and FSH have different releasing mechanisms in this species. (Endocrinology92: 1318, 1973)