The purpose of this study was to examine the functional interaction between endogenous opioid and cannabinoid receptor systems in the caudate putamen and nucleus accumbens. We therefore examined by autoradiography the functional activity and density of µ-, κ- and δ-opioid receptors in both brain regions of cannabinoid CB1 receptor knockout mice. Functional activity was estimated by measuring agonist-stimulated [35S]GTPγS binding. Results showed that deletion of the CB1 cannabinoid receptor markedly increased κ-opioid (50%) and δ-opioid (42%) receptor activities whereas no differences were found in µ-opioid receptor in the caudate putamen. In contrast, binding autoradiography showed a similar density of µ-, κ- and δ-opioid receptors between mutant and wild-type mice. No differences were found in densities or activities of µ-, κ- and δ-opioid receptors between mutant and wild-type mice in the nucleus accumbens. Taken together, our results revealed that deletion of CB1 cannabinoid receptors produced a pronounced increase in the activity of κ- and δ-opioid receptors in the caudate putamen. This endogenous interaction between opioid and cannabinoid receptors may be relevant to further understand a variety of neuroadaptative processes involving the participation of opioid receptors, such as motor behaviour, emotional responses and drug dependence.