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Department of Physiology, Queen's University, Kingston, Ontario, Canada

(Received 11 November 1974)

Gonadal steroids in the circulation are thought to act on adenohypophysial luteinizing hormone (LH) and hypothalamic LH releasing hormone (LH-RH) to reduce their secretion. The data on which this concept is based were obtained with relatively insensitive bioassay methods (Piacsek & Meites, 1966). Contrary to expectation, we found that both serum and hypothalamic LH-RH had fallen 3 weeks after castration (Shin, Howitt & Milligan, 1974). This is not consistent with the conventional feedback concept of hypothalamic LH-RH, but has recently received confirmation (Eskay, Oliver, Grollman & Porter, 1974; Ferland, Coté & Labrie, 1974).

To clarify better our understanding of the factors that influence LH-RH secretion, the time course of changes in LH and LH-RH after castration have been examined in more detail.

Male Sprague–Dawley rats were handled as described previously (Shin et al. 1974). Hypothalami were extracted according