The opioid and cannabinoid systems share a multiplicity of effects at biochemical and behavioural levels. Both opioid and cannabinoid agonists inhibit adenylyl cyclase activity, and increase the activity of MAP kinase pathways. They also produce similar changes in the permeability of several ion channels, and inhibit neurotransmitter release. Anatomical studies show that CB1 cannabinoid receptors and mu opioid receptors have a similar distribution in the central nervous system and are localised in the same neurons. Behavioural effects induced by opioids and cannabinoids include hypolocomotion, hypothermia, antinociception, and reward. Reciprocal interactions between the cannabinoid and opioid systems have been demonstrated for several responses including nociception, the development of tolerance and dependence, and the rewarding properties. In this review, the literature concerning the participation of the opioid system in the acute and chronic effects induced by cannabinoids will be discussed.