The specificity of the gonadotropin short-loop feedback mechanism was investigated in castrated rabbits by blood sampling from an indwelling jugular venous catheter for follicle-stimulating hormone (FSH) After the i.v. administration of purified human luteinizing hormone (hLH). Although such hLH administration results in a decrease of rabbit LH, no decrease in rabbit FSH was seen in these experiments. This specificity of LH feedback is compatible with the site of feedback being at the pituitary level or at the hypothalamic level provided 2 separate LH and FSH releasing factors exist.