1 The effect on arterial blood pressure of intravenous injections of naloxone (200 mug) was examined in cats anaesthetized with chloralose. Usually these injections have no effect on blood pressure unless morphine or opioid peptides have been injected, when they produce a pressor response with tachycardia.2 It was found that these injections produced a pressor response with tachycardia after a combination of two or more of the following surgical procedures: (1) tying sinus nerves, (2) removing stellate ganglia, (3) cutting vagi, (4) evisceration.3 The pressor responses obtained in these conditions are taken as evidence that such procedures induce the release of endogenous opioid peptides.4 The pressor responses to naloxone were greatest when all four surgical procedures had been performed and were then due to adrenaline secretion, evoked centrally by a sympathetic discharge to the adrenals.5 If either the stellate ganglia or the viscera were left intact, but the remaining three surgical procedures performed, then the pressor responses to naloxone were due to a sympathetic discharge to adrenals and to blood vessels.6 In cats that had received a subcutaneous injection of morphine (2 mg/kg) the adrenals played a minor role in the pressor responses to naloxone, unless the four surgical procedures had been performed. Then the adrenals became entirely responsible for them.7 The opioid peptides released after the surgical procedures may be enkephalins or the C-fragment of lipotropin (beta-endorphin). The stimulus for their release may be interruption of afferent sensory pathways from viscera or the ;stress' associated with the surgical procedures.