SUMMARYThe duration of action of 100μg of the synthetic gonadotrophin releasing hormone, the decapeptide LH/FSH‐RH, was compared in three subjects after intravenous, intramuscular and subcutaneous administration, given in random order at 7–10 day intervals. The levels of serum LH and FSH were followed over 9 hr. Each route of administration was equally effective in promoting LH and FSH release, and the magnitude of the responses and their time course were the same in each case. Two of these subjects then received 2 mg of the pure material dissolved in 0±5 or 1 ml of 0±9% NaCl by intranasal application. Although definite LH and FSH responses were seen, this route was less effective in promoting gonadotrophin release despite the higher dose used. In three further subjects blood sampling was carried out at 10 min intervals for 4 hr, following i.v., i.m. or s.c. injection of LH/FSH‐RH; pulsatile release of LH and FSH was observed in each subject but the fluctuations in LH and FSH were asynchronous. The therapeutic implications of subcutaneous and intranasal administration of the material are d