Experimental and clinical evidence indicates that high risk for sudden death is significantly correlated with post-myocardial infarction depression in two "markers" of vagal activity, heart rate variability and baroreflex sensitivity. The present experiments were designed to answer some of the questions generated by those findings. In 33 anesthetized cats, the neural activity of single cardiac vagal efferent fibers was recorded in control conditions and after injection of phenylephrine (n = 33), before and during a 1-hour coronary artery occlusion (CAO) (n = 17), and before and after removal of the left stellate ganglion (n = 16). In the first minute after CAO, vagal activity increased by 35% from 1.66 +/- 0.37 to 2.57 +/- 0.62 impulses/sec (p less than 0.01); despite a slight decline, it remained for the entire CAO above the control values, to which it returned after CAO release. Of 17 cats, ventricular fibrillation occurred in nine (susceptible) and eight survived (resistant). Resistant and susceptible cats had different reflex vagal responses to CAO. Whereas the resistant cats had a 48% (p less than 0.01) increase by the second minute of CAO, susceptible cats had no change (-18%, p = NS) in vagal activity. These differences were independent of blood pressure changes. The increase in vagal efferent activity in response to the blood pressure rise induced by phenylephrine (baroreceptive reflex) was more marked in the resistant cats compared with the susceptible cats (+246 +/- 66% versus +80 +/- 14%, p less than 0.025). Just before the injection of phenylephrine, vagal activity was not different between resistant and susceptible cats (1.58 +/- 0.35 versus 1.48 +/- 0.30 impulses/sec, p = NS). In 16 cats, left stellectomy increased cardiac vagal efferent activity by 75% (p less than 0.01), and the reflex vagal activation secondary to phenylephrine was further enhanced (from 2.2 +/- 0.4 to 4.7 +/- 0.7 impulses/sec, p less than 0.001). These data demonstrate that 1) cardiac vagal efferent activity increases in response to acute myocardial ischemia--much more so among the animals destined to survive, 2) before CAO, susceptible and resistant animals can be identified by the vagal response to blood pressure increase (assessed clinically by baroreflex sensitivity) and not by tonic vagal activity (assessed clinically by heart rate variability), and 3) the findings with left stellectomy support the hypothesis that vagal activity decreases after myocardial infarction because of an increase, secondary to abnormal stretch of the cardiac mechanoreceptors, in cardiac sympathetic afferent traffic, which exerts a tonic restraint on vagal outflow.