The acute effects of cannabinoid drugs on the synthesis of noradrenaline, dopamine, and serotonin (5-HT) were assessed, simultaneously, using the accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo. Treatment (1 h, i.p.) with Delta(9)-tetrahydrocannabinol (THC, 5, 10, and 20 mg/kg) and the cannabinoid receptor agonist WIN 55,212-2 (WIN, 2 and 4 mg/kg) increased dopa/noradrenaline synthesis (40-70%) in various brain regions enriched in this neurotransmitter (e.g., cerebral cortex, hippocampus, hypothalamus). In most brain regions, the content of noradrenaline was reduced by cannabinoid drugs (27-66%). For the effects of WIN (2 and 4 mg/kg), an inverse correlation ( r=-0.61, P=0.036) was obtained between the accumulation of dopa and the content of noradrenaline in the hypothalamus. The stimulatory effect on dopa accumulation induced by THC was antagonized by the selective CB(1) receptor antagonists SR141716A and AM 281 (10 mg/kg). In contrast, THC and WIN decreased the synthesis of dopa/dopamine in the corpus striatum (16-37%) and that of 5-HTP/5-HT (20-35%) in brain regions enriched in 5-HT (e.g., cerebral cortex and hippocampus). These inhibitory effects of THC and WIN were also antagonized by AM 281 and/or SR141716A. THC did not alter the content of 5-HT or dopamine in the brain. The effects may be related to the activation of presynaptic inhibitory cannabinoid CB(1) receptors located on the neurones themselves (serotonin) and on facilitatory (dopamine) and inhibitory interneurones (noradrenaline).