In a warm environment at ambient temperatures between 25 degrees and 38 degrees C (relative humidity 50%-60%) the relationship between sympathetic activity in cutaneous nerves (SSA) and pulses of sweat expulsion was investigated in five young male subjects. The SSA was recorded from the peroneal nerve using a micro-electrode. Sweat expulsion was identified on the sweat rate records obtained from skin areas on the dorsal side of the foot, for spontaneous sweating and drug-induced sweating, using capacitance hygrometry. Sweat expulsion was always preceded by bursts of SSA with latencies of 2.4-3.0 s. This temporal relationship between bursts of SSA and sweat expulsion was noted not only in various degrees of thermal sweating but also in the sweating evoked by arousal stimuli, or by painful electric stimulation. The amplitude of the sudomotor burst was linearly related to the maximal rate of increase of the corresponding sweat expulsion, the amplitude of the expulsion and the integrated amount of sweat produced for the duration of the expulsion. The results provide direct evidence that sweat expulsion reflects directly centrally-derived sudomotor activity.