Using heterologous radioimmunoassay systems, it has been possible to demonstrate a complete cross-reaction between partially purified preparations of guinea pig FSH and LH and the corresponding rat gonadotrophìns. On this basis, radioimmuno-assays for these 2 gonadotrophìns have been established and they have been used in order to study changes in blood hormonal levels during the estrous cycle. There is a serum LH peak during estrus shortly before ovulation. At ovulation, serum LH levels show a smaller increase which is maintained for 1 or 2 days. The changes in serum FSH levels do not systematically coincide with those of LH. A parallel study of the changes in serum progesterone and estradiol was also performed. The levels of estrogen increased during the pre-ovulatory period, reaching a maximum shortly before the pre-ovulatory LH peak. Progesterone does not increase until shortly thereafte