The inotropic effect on the heart of stimulating the vagus in the dog, duck and toad.

1. The chronotropic and inotropic effects of stimulating the vagus on the hearts of the dog, duck and toad were studied.2. The maximum rate of rise of pressure in the left ventricle (dP/dt max) measured at a constant heart rate and mean aortic pressure was used as an index of the inotropic changes.3. The sensitivity of dP/dt max as an index of inotropic changes brought about by stimulating the vagus was established in the toad where a 49% reduction in heart rate was associated with a 30% reduction in dP/dt max.4. In the dog stimulation of the vagus resulted in a reduction in heart rate of 38% and only a small reduction in dP/dt max of 6%.5. Results similar to those found in the dog were obtained in the duck where the reduction in heart rate of 44% was associated with reduction in dP/dt max of only 3%.6. It is concluded that the vagus has only a small and negligible negative inotropic effect on the ventricles of the dog and duck.