The interaction between small doses of dopamine and droperidol on effective renal plasma flow was studied in dogs. Small doses of dopamine are known to produce renal vasodilatation by a dopaminergic mechanism and droperidol, a selective dopamine antagonist used in neuroleptanaesthesia, may attenuate this response. Effective renal plasma flow was measured non-invasively using 125I-hippuran. A 20-min infusion of dopamine 2 microgram kg-1 min-1 significantly enhanced effective renal plasma flow (mean increase of 16%; P < 0.05) in anaesthetized dogs. This effect was abolished (mean decrease 6.6%) by droperidol 0.2 mg kg-1 administered at the commencement of anaesthesia. It is concluded that droperidol reduces the renal vasodilatation induced by dopamine in anaesthetized dogs.