Three types of opioid receptors have been cloned: mu, delta, and kappa. Opioid receptors are seven transmembrane G-protein coupled receptors. They share a high degree of homology and are most divergent at the N- and C-termini. Activation of mu opioid receptors leads to a decrease in neuronal excitability. Most actions of exogenous opioids, such as morphine, are mediated through the μ-opioid receptor, including analgesia, tolerance and reward. In general, opioids modulate numerous central and peripheral processes including pain perception, neuroendocrine secretion and the immune response. The opioid signal is transduced from receptors through G proteins to various different effectors. Subsequent to G protein activation, several effectors are known to orchestrate the opioid signal. For example, activation of opioid receptors increases phosphatidylinositol turnover, activates K+ channels and reduces adenylyl cyclase and Ca++ channel activities.