Intraperitoneal injection of delta9-THC (7.5 mg/kg) in rats made tolerant to morphine by s.c. implantation of morphine pellets had a much greater analgesic effect than in placebo pellet plus delta9-THC treatment. To investigate whether this was due to some change in cannabinoid receptor levels and/or expression induced by chronic morphine, we designed this autoradiographic binding study coupled with in situ hybridization on sagittal sections of the treated rat brains. Binding showed a significant increase in CB1 receptor density (15%) specifically in the caudate-putamen, in parallel with a significant enhancement of CB1 mRNA in the same area (20%). We suggest that morphine chronic treatment leads to a functional modulation between the opioid and cannabinoid systems at least for analgesia in a specific area, in this case the striatum.