Recent studies have demonstrated that the pharmacological tolerance observed after prolonged exposure to plant or synthetic cannabinoids in adult individuals seems to have a pharmacodynamic rather than pharmacokinetic basis, because down-regulation of cannabinoid receptors was assessed in the brain of cannabinoid-tolerant rats. In the present study, we have examined the time-course of cannabinoid receptor down-regulation by analyzing cannabinoid receptor binding, using autoradiography, and mRNA expression, using in situ hybridization, in several brain structures of male adult rats daily exposed to delta9-tetrahydrocannabinol (delta9-THC) for 1, 3, 7, or 14 days. With only the exception of a few number of areas, most of the brain regions exhibited a progressive decrease in cannabinoid receptor binding. Two facts deserve to be mentioned. First, the pattern of this down-regulation process presented significant regional differences in terms of onset of the decrease and magnitude reached. Second, the loss of cannabinoid receptor binding was usually accompanied by no changes in its mRNA expression. Thus, some structures, such as most of the subfields of the Ammon's horn and the dentate gyrus in the hippocampus, exhibited a rapid (it appeared after the first injection) and marked (it reached approximately 30% of decrease after 14 days) reduction of cannabinoid receptor binding as a consequence of the daily delta9-THC administration. However, no changes occurred in mRNA levels. Decreased binding was also found in most of the basal ganglia, but the onset of this reduction was slow in the lateral caudate-putamen and the substantia nigra (it needed at least three days of daily delta9-THC administration), and, in particular, in the globus pallidus (more than 3 days). The magnitude of the decrease in binding was also more moderate, with maximal reductions always less than 28%. No changes were seen in the entopeduncular nucleus and only a trend in the medial caudate-putamen. However, the decrease in binding in some basal ganglia was, in this case, accompanied by a decrease in mRNA levels in the lateral caudate-putamen, but this appeared after 7 days of daily delta9-THC administration and, hence, after the onset of binding decrease. In the limbic structures, cannabinoid receptor binding decreased in the septum nuclei (it needed at least 3 days of daily delta9-THC administration), tended to diminish in the nucleus accumbens and was unaltered in the basolateral amygdaloid nucleus, with no changes in mRNA levels in these last two regions. Binding also decreased in the superficial and deep layers of the cerebral cortex, but only accompanied by trends in mRNA expression. The decrease in binding was initiated promptly in the deep layer (after the first injection) and it reached more than 30% of reduction after 14 days of daily delta9-THC administration, whereas, in the superficial layer, it needed more than 3 days of daily delta9-THC administration and reached less than 30% of reduction. Finally, no changes in binding and mRNA levels were found in the ventromedial hypothalamic nucleus. In summary, the daily administration of delta9-THC resulted in a progressive decrease in cannabinoid receptor binding in most of the brain areas studied, and it was a fact that always occurred before the changes in mRNA expression in those areas where these existed. The onset of the decrease in binding exhibited regional differences with areas, such as most of the hippocampal structures and the deep layer of the cerebral cortex, where the decrease occurred after the first administration. Other structures, however, needed at least 3 days or more to initiate receptor binding decrease. Two structures, the entopeduncular nucleus and the ventromedial hypothalamic nucleus, were unresponsive to chronic delta9-THC administration, whereas others, the medial caudate-putamen and the basolateral amygdaloid nucleus, only exhibited trends.