An attempt has been made to elucidate the cause of hyperhidrosis in dysautonomia by testing sweat gland responsiveness to local application of heat. Heat was applied to the volar surface of the forearm with temperatures of 41.5 C and less for one to ten minutes. For each subject, the minimal duration of heating at the lowest temperature capable of evoking sweating was determined (threshold). Thresholds were significantly lower in all subjects with dysautonomia. Since one control subject, the father of a dysautonomia subject, had the highest threshold, these tests do not permit detection of the heterozygote. A reasonable explanation for enhanced sweating reactivity to local heat application is an increased excitatory state of reflex centers. These patients appear permanently on the verge of generalized sweating likely to result from minimal additional stimulation.