The endocannabinoid system and the cannabinoid CB(1) receptors are involved in the development of ethanol tolerance and dependence. This study aimed to investigate the in vivo sensitivity of a CB(1) receptor agonist (WIN 55,212-2) modulating the synthesis of 3,4-dihydroxy-phenylalanine/dopamine/noradrenaline (DOPA/DA/NA) and that of 5-hydroxy-tryptophan/serotonin (5-HTP/5-HT) in rat brain after ethanol treatment and withdrawal. In control rats, WIN 55,212-2 (4 mg/kg, i.p., for 1h), through a mechanism sensible to the CB(1) antagonist SR 141716A, increased the synthesis of DOPA/NA in a slice of brainstem containing the locus ceruleus (250%) and in the hippocampus (64%), and it reduced DOPA/DA synthesis in the striatum (47%). WIN 55,212-2 also decreased the synthesis of 5-HTP/5-HT in the locus ceruleus (43%), hippocampus (35%) and striatum (35%). In the locus ceruleus of ethanol-treated rats, the stimulatory effect of WIN 55,212-2 on DOPA/NA synthesis was abolished (acute treatment) or markedly attenuated (53-55%, chronic treatment and withdrawal), whereas in the hippocampus this effect was reduced only in chronic ethanol-withdrawn rats (33%). In the striatum of ethanol-treated rats (acute, chronic and withdrawal), the inhibitory effect of WIN 55,212-2 on DOPA/DA synthesis was completely blunted or markedly reduced. Similarly, the inhibitory effect of WIN 55,212-2 on 5-HTP/5-HT synthesis was reduced or abolished in the three brain regions after chronic ethanol and during withdrawal. These results indicate that treatment with ethanol in rats induces a functional desensitization of CB(1) receptors modulating the synthesis of brain monoamines.