(1) Micro-electrode recordings were made of multi-unit sympathetic activity in skin or muscle branches of the median nerve at the elbow and the peroneal nerve at the fibular head in 69 healthy subjects. In some recordings changes in skin resistance and pulse plethysmograms were monitored within the receptive field of the impaled fascicles. Conduction velocities in postganglionic sympathetic fibres were measured either directly in double nerve recordings (9 subjects) or indirectly by determining the latency of either of two sympathetic reflexes. For skin nerve sympathetic activity (SSA) the excitatory reflex response to an electrical skin stimulus was used and for muscle nerve sympathetic activity (MSA) the inhibitory baroreflex response to single arterial pulse waves. (2) For MSA, reflex latencies were 0.90--1.13 s in median and 1.22--1.54 s in peroneal nerve recordings. Corresponding latencies for SSA were 0.48--0.66 s and 0.72--0.91 s, respectively. Reflex latency showed a positive correlation with body height and with length of the extremity recorded from. From the correlation with extremity length, average conduction velocities for MSA in median and peroneal nerves were calculated to be 0.74 and 1.11 m/s, respectively. Corresponding figures for SSA were 1.69 and 1.16 m/s. (3) With direct determinations of conduction velocities average values for MSA (comprising mainly vasoconstrictor fibres) in median and peroneal nerves were 0.72 and 1.09 m/s, respectively. With direct SSA determinations, sympathetic bursts containing only vasoconstrictor impulses (giving plethysmographic responses but no changes of skin resistance) had lower conduction velocities than bursts containing only sudomotor impulses (giving electrodermal but no plethysmographic responses) or a mixture of sudomotor and vasoconstrictor impulses. Average values (0.77 and 1.27 m/s, respectively) were similar in median and peroneal recordings. (4) It is concluded that determination of reflex latency is a useful indirect measure of conduction velocity in sympathetic postganglionic fibres.