The long distance transport of sheep before slaughter is at present a very important topic in animal welfare. However, Modulation of opioid receptors can be influenced by chronic treatment with opioid agonists and antagonists (Blanchard, and Chang, 1988). Similarly, opioid receptors can be up or down-regulated by stressful stimuli such as restraint, electric footshock or social isolation and housing (Zeman et al., 1988 and Zanella et al., 1991). The present study was carried out to assess the effects of transport stress on the properties of one class of opioid peptide receptor in the brain of sheep after transport stress. Opioid peptides such as β-endorphin are released by the central nervous system during application of stresses such as transport. They are believed to exert analgesic properties and their effectiveness depends partly on the concentration (Bmax) and affinity (Kd) of their receptors. µ-Opioid receptors are found in various brain regions and are selective for endorphins and similar peptides.