Luteinizing hormone (LH) follicle stimulating hormone (FSH) and testosterone responses to prolonged infusion of gonadotropin releasing hormone (GnRH) in 6 normal human males are reported. Synthetic GnRH was administered by iv bolus injections of .375 .75 and 1.5 mcg/kg and constant infusions at the rate of .125 .25 .5 1 and 2 mcg/kg/hour. Continuous application of low doses induced a marked rise of LH elevation of testosterone and a distinct delayed increase of FSH secretion. The findings of a biphasic secretion of LH during GnRH infusion may be explained by the hypothesis of 2 pools of LH in the human pituitary 1 quickly available by higher doses and the other only by prolonged stimulation with much lower doses. It is suggested that treatment with depot preparations or long-acting analogs will probably have the same or better effect and will be more practical.