Long-term cannabinoid administration produces region-dependent CB 1 receptor desensitization and down-regulation. This study examined the time course for normalization of CB 1 receptors and G-protein activation using 35 S]GTP␥S binding were decreased in both regions 1 day after treatment. WIN55,212-2-stimulated G-protein activation in striatum/GP returned to control level at 3 days after cessation of treatment with either drug but did not return to control level in hippocampus until 14 days. CB 1 receptor binding did not recover to control levels until day 7 or 14 after treatment in striatum/GP and hippocampus, respectively. The mechanism of CB 1 binding site down-regulation was investigated after long-term ⌬ 9 -THC treatment. Analysis of CB 1 receptor mRNA in hippocampus and striatum/GP showed that transcriptional regulation could not explain prolonged recovery rates from CB 1 receptor down-regulation. In contrast, CB 1 receptor protein, as determined by immunoblot analysis, matched the down-regulation and recovery rates of CB 1 receptor binding sites relatively closely. These data demonstrate that cannabinoid-induced decreases in CB 1 receptor function persist for relatively long time periods after cessation of long-term drug treatment and that CB 1 receptor signaling recovers more quickly in striatum/GP than hippocampus. Moreover, down-regulation of CB 1 receptor binding sites does not seem to result mainly from transcriptional regulation, suggesting that adaptive regulation of CB 1 receptors in brain primarily occurs at the protein level.