Studies were undertaken to determine if the number of hypophyseal receptors for GnRH changes at the time of the preovulatory surge of LH in ewes. Concentrations of LH, FSH, progesterone, and estradiol in serum and concentrations of LH and FSH in pituitary were measured. The content of GnRH in the hypothalamus was also determined. Estrus was synchronized in 35 cross-bred ewes by injecting prostaglandin F2 alpha (PGF2 alpha) at 0 and 4 h (7.5 mg each, im) on day 14 of a naturally occurring estrous cycle, followed 30 h later by the injection of estradiol (25 micrograms in safflower oil, im). Five ewes were killed at each of the following times relative to the first injection of PGF2 alpha: 0, 24, 32, 44, 50, 56 and 96 h. Blood samples were collected throughout the course of the experiment. Concentrations of progesterone in serum decreased markedly by 8 h after PGF2 alpha and were uniformly undetectable (less than 300 pg/ml) by 34 h. Concentrations of estradiol in serum increased after the injection of estradiol and returned to basal values 10 h later. Surges of LH, which were usually coincident with surges of FSH, occurred between 43 and 53 h. Concentrations of both LH and FSH in the pituitary declined after the LH surge. There were no significant changes in the amount of GnRH contained in the preoptic area, the median eminence, or the hypothalamus. The number of receptors for GnRH increased at 24 and 32 h compared to the 0 h value and remained elevated at 44 and 50 h. After the LH surge (56 h), the number of GnRH receptors declined and at 96 h was not different from the number measured at 0 h. Since an increase in the number of receptors will result in the formation of more receptor-hormone complex and may lead to an augmented response, these data suggest that an increase in the number of hypophyseal receptors for GnRH may contribute to the preovulatory LH surge in ewes.