Muscle sympathetic nerve activity (MSA) was quantitatively analyzed in amyotrophic lateral sclerosis (ALS) by using microneurography. Studies were conducted in 10 ALS patients (2 males, 8 females; mean age 56.4 SD 17.3 years; range 31-77 years). The duration of the disease after onset of symptoms was from 0.8 to 2.5 years. They were classified as a type of classical ALS having mild degree of bulbar signs, an ability to walk by themselves and no subjective breathing impairment. Control groups were selected from 16 healthy age-matched volunteers (8 males, 8 females; mean age 52.9, SD 15.5 years; range 29-76 years). MSA was recorded directly from peroneal nerve fascicles at the popliteal fossa by means of tungsten microelectrodes. The changes in MSA, blood pressure and heart rate were analyzed in the ALS groups and controls laid at recumbent and 30 degrees head-up tilted positions. The parasympathetic functions were evaluated by CVR-R and baroreflex latency. MSA bursts/minute, bursts/100 heart rate and total MSA at recumbent position were significantly higher (p less than 0.01), but their changes at 30 head-up tilted position were slightly lower in the ALS groups compared with the controls. Furthermore, a linear correlation between age of the subjects and MSA in the controls was not found in ALS groups. There were no differences between both groups in blood pressure and heart rate at the two positions. The parasympathetic function was normal in ALS groups. In ALS, sympathetic hyperfunction independent of cardiovascular systems was found especially at the recumbent position. These findings were considered specific to ALS, but the etiology was unclear.