The present study tested whether exogenous gonadotrophin-releasing hormone ( GnRH) and luteinising hormone (LH) can stimulate LH and testosterone secretion in dogs chronically treated with a GnRH superagonist. Twenty male adult dogs were assigned to a completely randomised design comprising five groups of four animals. Each dog in the control group received a blank implant (placebo) and each dog in the other four groups received a 6-mg implant containing a slow-release formulation of deslorelin (d-Trp(6)-Pro(9)-des-Gly(10)-LH-releasing hormone ethylamide). The same four control dogs were used for all hormonal challenges, whereas a different deslorelin-implanted group was used for each challenge. Native GnRH (5 mu g kg(-1) bodyweight, i.v.) was injected on Days 15, 25, 40 and 100 after implantation, whereas bovine LH (0.5 mu g kg(-1) bodyweight, i.v.) was injected on Days 16, 26, 41 and 101. On all occasions after Day 25-26 postimplantation, exogenous GnRH and LH elicited higher plasma concentrations of LH and testosterone in control than deslorelin-treated animals (P < 0.05). It was concluded that, in male dogs, implantation of a GnRH superagonist desensitised the pituitary gonadotrophs to GnRH and also led to a desensitisation of the Leydig cells to LH. This explains, at least in part, the profound reduction in the production of androgen and spermatozoa in deslorelin-treated male dogs.