Single iv injections of a rabbit antiserum to synthetic LHRH promptly suppressed serum LH and FSH concentrations in ovariectomized rhesus monkeys. Gonadotropin levels remained depressed for 10-21 days, the approximate duration of enhanced LHRH binding activity in the circulation. Doses of LHRH antiserum sufficient to reduce tonic gonadotropin secretion did not modify the time course or magnitude of estrogen-induced gonadotropin surges. These negative findings, however, cannot be interpreted to signify that such surges are not caused by a release of LHRH.