Failure of autonomic functions in diabetes has been investigated. Cardiovascular reflexes, and thermoregulation during heat exposure, were studied in 11 diabetic patients. Five had clinical signs of sensorimotor neuropathy and 6 served as age- and sex-matched controls. The subjects were studied by the following tests: orthostatic test, Valsalva, beat-to-beat variation during normal and forced respiration, and analysis of urinary catecholamines. Thermoregulation was measured for 80 minutes, at an ambient temperature of 37 degrees C, in a gradient layer direct calorimeter measuring evaporative heat losses with a precision of +/- 1 watt. The cardiovascular tests showed a relationship between the sensorimotor defects and the autonomic dysfunctions. The beat-to-beat variation was found to be the most sensitive test, the Valsalva and orthostatic test being respectively less so. However, the diabetic patients' autonomic disorders were not associated with a significant decrease in evaporative heat losses. The internal temperature (esophageal) at which sweating began (set point) was higher in some of the neuropathic diabetics than in the controls. Once the set point had been reached, however, the increase in evaporative heat losses was greater in diabetics than in controls. These findings can be accounted for by zones of compensatory hypersweating in patients with polyneuropathy. The changes in evaporative heat loss pattern may represent an early alteration in autonomic diabetic neuropathy.