Changes in Gonadotropin-Releasing Activity of the Hypothalamus During an Ovulatory Cycle of the Hen *

ABSTRACT The gonadotropin-releasing activity of the stalk-median eminence (SME) of hens at various times during the period between ovulations of the initial ovum and the second ovum (C2) of a clutch was measured using an in vitro incubation system and a bioassay based on the 32P-uptake by chick testes. The gonadotropin-releasing activity of the SME changes during the ovulatory cycle, with peaks 21 and 8-11 hr. before ovulation (CJ. The gonadotropins released from the incubated pituitaries by the SME of hens 11 hr. before C2 ovulation showed a greater ovulation-inducing activity on hens than those released by the SME of hens 21 hr. before C2 ovulation, while the activity for the 32P-uptake by chick testes was almost equal. POULTRY SCIENCE 53: 1555-1560, 1974 THE existence of a hypothalamic sub-stance regulating the release of gona-dotropins from the pituitary of the chicken has been suggested from the facts that the chicken hypothalamic extracts have an activ-ity to stimulate the release of ovulation-in-ducing hormone from the hen's pituitary i