The changes in plasma levels of LH and FSH were examined after treatment of intact and ovariectomized females ferrets with a long-acting analogue of gonadotrophin releasing factor (Gn-RF) (D-Ser(But)6-LH-releasing hormone (1-9)-nonapeptide ethylamide; Hoe 766) either as a single intravenous injection or daily for 6 days. The responses were compared with those induced by daily injections of Gn-RF or 0.9% NaCl (w/v). Treatment with Gn-RF consistently induced rises in both LH and FSH release with peak levels of both hormones being reached 20 min after injection and being of similar size from day to day in individual animals. Thereafter, the gonadotrophin levels declined rapidly to approach basal values by the end of each sampling period. Treatment with Hoe 766, however, produced very high values on day 1 of treatment, with LH being raised for 10-12 h and FSH for up to 24 h. Subsequent injections, on the other hand, produced an abbreviated LH response of similar size to that induced by Gn-RF and little, if any, FSH response. In ovariectomized ferrets, Hoe 766 induced a variable LH response and little FSH response at any time. In addition, basal FSH levels in the first three samples taken on each day from day 2 onwards tended to decline markedly in all of the Hoe 766-treated animals, an effect not seen in Gn-RF or 0.9% NaCl-treated controls.