The sheep is a highly social animal and isolation from others members of the same species causes acute emotional stress (Kilgour & De Langen, 1970). Emotional or psychological stress rather than physical stress appears to activate the hypothalamo-pituitary-adrenal system (Mason, 1968). As part of a study of the effect of stress on reproduction, we have investigated the effect of temporary isolation of sheep on the release of β-endorpbin and concomitant secretion of LH. Previous reports have shown that isolation causes β-endorphin release from the pituitary (Matven, 1987 and Al-Gahtani & Rodway, 1991), but there is little information available on the effect of isolation stress on LH release. The purpose of this work was to examine the temporal relationship between β-endorphin release and LH secretion in sheep during isolation.