Hagbarth & Vallbo made the first direct microneurographic recordings of postganglionic sympathetic nerve discharges in man (32). Since then per cutaneously inserted microelectrodes have been used extensively for the study of human sympathetic function. Apart from a recent study of sympathetic activity in trigeminal nerve branches (44), the microneurographic exploration of sympathetic mechanisms has bcen confined to nerves of the extremities. Two different types of sympathetic outflow have been recognized in multifi ber recordings: muscle nerve sympathetic activity (MSA) and skin nerve sympathetic activity (SSA). The former is dominated by vasoconstrictor signals; the latter is a mixture of sudomotor and vasoconstrictor and probably sometimes includes pilomotor and vasodilator impulses. Thus only two sub divisions of the anatomically multifaceted sympathetic nervous system are accessible to study. Despite this limitation, the results have allowed a number of conclusions, not only about peripheral sympathetic function but also about reflex patterns and hence general principles of sympathetic regulation.