The dopamine D2-receptor antagonist sulpiride decreases spontaneous locomotor velocity of planarians (pLMV) in an enantiomeric-selective and dose-dependent manner and is significantly attenuated by UV light (254 and 366 nm). We now report that amphetamine (10 microM) produced the opposite effect and was also reversed by UV light. These findings strengthen the hypothesis that the effects of dopaminergic ligands and UV light on pLMV relate to interaction with neurotransmitter transduction process(es).