1. The climbing behaviour in mice was used for studying possible interaction(s) of d-LSD with dopamine receptors. 2. Doses of d-LSD ranging from 0.25 to 2.5 mg/kg injected intra-peritoneally constantly inhibited the climbing behaviour. 3. In contrast, when similar doses of d-LSD were injected 10 min before apomorphine (5 mg/kg), a constant potentiation of the apomorphine-induced climbing was observed. 4. Subsequent experiments performed with a neuroleptic (haloperidol) or a serotonin precursor (5-OH-tryptophan) compared to those of d-LSD with and without apomorphine would indicate that d-LSD alone displays typical serotoninergic syndrome (including inhibition of the climbing), whereas in the presence of apomorphine, an interaction at presynaptic receptors may possibly modulate dopaminergic activity.