1. In conscious rabbits, intravenous morphine (3 mg/kg) caused hypertension, bradycardia, hyperglycaemia and sedation. These changes were accompanied by large increases in plasma adrenaline and smaller increases in plasma noradrenaline.
2. These effects of morphine were prevented by intravenous naloxone, demonstrating their dependence on stimulation of opiate receptors.
3. Pretreatment with the antihistamines cimetidine and chlorpheniramine enhanced the morphine-induced rise in blood pressure, excluding a role for histamine release in the hypertensive action of morphine.
4. The centrally acting α2-adrenergic agonist clonidine prevented the morphine-induced hypertension and rise in plasma catecholamines, suggesting that these effects are exerted via central pathways. Clonidine alone reduced blood pressure and heart rate and produced hyperglycaemia.
5. α-Adrenergic blockade with phenoxybenzamine reduced the increase in blood pressure after morphine, although the increase in plasma catecholamines was augmented.
6. Pentobarbitone anaesthesia prevented the morphine-induced cardiovascular changes, the increase in plasma catecholamines and the hyperglycaemia.
7. These findings indicate, that in conscious rabbits, morphine induces hypertension by stimulation of opiate receptors leading to increased sympatho-adrenal activity. The hyperglycaemia appears to be in response to secretion of adrenaline. These effects probably result from a central action of morphine.