We studied the influence of respiration on muscle sympathetic and cardiac vagal activities in twenty conscious, healthy young adult subjects. Efferent post-ganglionic muscle sympathetic activity was measured directly with electrodes inserted percutaneously into a peroneal nerve, and vagal cardiac activity was measured indirectly from electrocardiographic changes of heart period. Muscle sympathetic activity waxed and waned with respiration; maximum activity occurred at end-expiration and minimum activity occurred at end-inspiration. Voluntary control of breathing did not alter the time course or magnitude of muscle sympathetic outflow. Spectral analyses showed that respiratory periodicities were present in sympathetic and vagal records. Average power at frequencies below respiratory frequencies exceeded or equalled that at respiratory frequencies in both muscle sympathetic and vagal cardiac records. A cardiac periodicity was present and conspicuous in muscle sympathetic recordings in all but one subject. Diastolic arterial pressure increased during inspiration and decreased during expiration. Heart period and muscle sympathetic activity paralleled each other and were related reciprocally to changes of diastolic pressure. Brief reductions of carotid baroreceptor afferent traffic provoked by neck pressure were more effective in increasing sympathetic activity in expiration than inspiration. We conclude that quiet respiration is associated with parallel phasic changes in activity of medullary vagal cardiac and spinal muscle sympathetic motonuclei in man; spontaneous activity and susceptibility to excitation or inhibition by autonomic inputs are greater in expiration than inspiration. Substantial power is present in both muscle sympathetic and cardiac vagal recordings at frequencies below respiratory frequencies.