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Department of Physiology, The Medical School, University of Birmingham, Birmingham B15 2TJ

1. A study has been made of eighty-four cells in the cat's nucleus ambiguus whose axons projected to the cardiac (seventy-four) and pulmonary (ten) branches of the right vagus. Their axonal conduction velocities were all in the range of B fibres (2·8-15·5 m/sec).

2. Pulmonary branch projecting neurones were usually spontaneously active (nine out of ten) and fired in phase with inspiration. Their activity showed no pulse modulation.

3. Ten cardiac branch projecting neurones had properties indistinguishable from those of pulmonary branch projecting neurones. Inspiratory-firing cells projecting to either branch are believed to be bronchoconstrictor in function.

4. The remaining sixty-four cells that projected to the cardiac branch had properties expected of cardioinhibitory neurones. Most (fifty-four) were silent until activated by ionophoresis of excitant amino acids. All showed an expiratory discharge when active, and of twenty-seven tested twenty-three showed a cardiac modulation of their discharge. When the aortic baroreceptors were denervated, the cardiac rhythm was always abolished reversibly by carotid occlusion.

5. Ionophoretic activation of expiratory firing (presumed cardioinhibitory) cells slowed the heart (fifteen out of eighteen neurones tested). Excited inspiratory-firing cells never had this effect (eleven tested).

6. Both types of neurones were found in the nucleus ambiguus, but presumed cardioinhibitory cells tended to be found more caudally and ventrally than presumed bronchoconstrictor neurones.

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