The effect of morphine on vagal inhibition of the heart.

The action of morphine (0.1 to 100 mg./kg.) on the cardiac slowing produced by stimulation of the right vagus nerve varied in the four species investigated. In the guinea-pig, morphine had no inhibitory effect and in the cat the effect was small, but in the rat and the rabbit stimulation of the vagus slowed the heart much less after administration of morphine than before. Further, in the last two species morphine delayed the onset of cardiac slowing. Nalorphine, which given alone had a slight morphine-like action, partly reversed the effect of morphine.