Substance P (SP),the first identified neuropeptide,has long been indicated to be involved in the vagal control of heart rate and cardiac functions.However,the mechanism by which SP modulate cardiac vagal activity is still not clear,and little is known about how SP alters the activity of preganglionic cardiac vagal neurons (CVNs) at the synaptic level.In the present study CVNs in the nucleus ambiguus (NA) of newborn rats were retrogradedly labeled with fluorescent tracer,and were examined using whole-cell voltage patch-clamp method in brainstem slices.We have demonstrated that SP (1 mol/L),while had no effect on the glutamatergic spontaneous excitatory synaptic currents (sEPSCs) of CVNs,significantly increased the frequency of both the GABAergic and glycinergic spontaneous inhibitory postsynaptic currents (slPSCs) of CVNs.In slices with respiratory-like rhythm,the inspiratory-related frequency increases of the GABAergic and the glycinergic slPSCs of CVNs were diminished by SP application.These results suggested that SP inhibit CVNs via augmentation of their inhibitory inputs and diminish the respiratory-related regulation of cardiac vagal activity,which might be related to the weakened cardiac vagal tone and heart rate variability in myocardial ischemia and hypertension.