1. The role of endogenous opioids on the reflex cardiovascular control of chronic uraemic patients was investigated.
2. The opiate antagonist naloxone administered intravenously caused a significant increase in the abnormal Valsalva manoeuvre ratio in nine chronic uraemic patients, but it had no effect in six diabetic patients with normal renal function, whose response to the Valsalva manoeuvre was similar to that of chronic uraemic patients. Naloxone had no effect in eight normal subjects.
3. The increase in the Valsalva ratio observed in uraemic patients was due to restoration of the parasympathetically mediated reflex bradycardia of the release phase of the manoeuvre.
4. Naloxone did not modify supine and standing blood pressure and heart rate in any group.
5. Endogenous opioids may be involved in the defective autonomic control of heart rate in uraemic patients.