The effect of stress-like concentrations of cortisol on oestradiol-induced change in LH secretion and GnRH receptor expression was evaluated in orchidectomized sheep (wethers). Twenty-four wethers were assigned at random to one of the four treatment groups in a 2×2 factorial design ( n=6 wethers/group). Wethers received cortisol (90 μg/kg/h; groups 2 and 4) or a comparable volume of cortisol delivery vehicle (groups 1 and 3) by continuous infusion for 48 h. During the final 24 h of infusion, wethers received oestradiol (6 ng/kg/h; groups 3 and 4) or oestradiol delivery vehicle (groups 1 and 2). The pattern of LH secretion was assessed during a 3-h period of intensive blood collection beginning 21 h after initiation of oestradiol infusion. Although neither cortisol nor oestradiol alone affected ( P>0.05) mean serum concentration of LH or LH pulse frequency, serum LH and the frequency of secretory episodes of LH were significantly reduced ( P<0.05) in wethers receiving cortisol and oestradiol in combination. Anterior pituitary tissue was collected at the end of the infusion period. Oestradiol increased ( P<0.05) tissue concentrations of GnRH receptor and GnRH receptor mRNA. Although cortisol alone did not affect ( P>0.05) basal concentrations of receptor or receptor mRNA, the magnitude of oestradiol-induced increase in GnRH receptor and GnRH receptor mRNA was significantly reduced in wethers receiving cortisol and oestradiol concurrently. Conversely, steady-state concentrations of mRNA encoding the LHβ and FSHβ subunits were increased ( P<0.05) in wethers receiving cortisol. These observations demonstrate that stress-like concentrations of cortisol act in concert with oestradiol to suppress LH secretion. In addition, cortisol blocks oestradiol-dependent increase in pituitary tissue concentrations of GnRH receptor and GnRH receptor mRNA.