Dopamine, a precursor of noradrenaline, is found in high concentrations in mammalian tissues (1). It has got many pharmacological actions similar to, but much weaker than adrenaline and noradrenaline. It produces a vasopressor response in dog and cat in doses 20 to 50 times higher than those of adrenaline (2, 3). Like adrenaline, the pressor response of dopamine is blocked by alpha adrenergic blocking agents and is con verted into a depressor response (4), but, unlike adrenaline and isoprenaline, the depressor response of dop amine is not blocked by conventional beta adrenergic blocking agents (4, 5). This response is also not blocked by antihistaminic, anticholinergic and ganglion blocking agents (4). Van Rossum (6), however, recently reported that spiramide and haloperidol block the dopamine depressor response. Haloperidol is structurally related to pethidine (7). The present study was, therefore, undertaken to investigate the blocking effect of pethidine and allied drugs, on the depressor response of dopamine. In addition, phenothiazines, which have a profile of action similar to that of haloperidol, have also been included in the study. Cats of either sex weighing from 2 to 4 kg were used. The animals were anaesthetized with pentobar bitone sodium (35 mg/kg intraperitoneally), bilaterally vagotomized and maintained on positive pressure artificial respiration. The blood pressure was recorded from a common carotid artery by means of a mercury manometer on a smoked paper. Femoral vein of one side was cannulated by a polythene tube for intravenous administration of drugs. Adrenergic a-blockers (dibenzyline 10 mg/kg or yohimbine hydrochloride 1 mg/kg) were used to obtain the depressor response of dopamine (50-100 leg/kg i.v.). Phenothiazines (chlorpromazine hydrochloride and fluphenazine hydrochloride) and pethidine hydrochloride and related drugs (morphine