The dopamine D2 receptor antagonist sulpiride decreases the spontaneous locomotor activity of Planaria in an enantiomeric-selective and dose-dependent manner. We now report that (-)sulpiride (0.1 microM)-induced decrease of planarian locomotor activity is significantly (P<0.05) attenuated by low-energy (366 nm) ultraviolet (UV) light and to a greater extent by high-energy (254 nm) UV light. The phenomenon offers a novel approach for studying dopamine D2 receptor transduction processes in a simple in vivo model.