The long-term course of the parasympathetic neuropathy in patients with chronic renal failure maintained on haemodialysis was investigated. Well-established tests of parasympathetic function (deep breathing test, atropine test) were performed in 45 patients who had been dialysed for periods ranging from 1 to 12 years and in 34 normal subjects. The responses to these tests were clearly compromised in dialysis patients but there was no relationship between duration of haemodialysis treatment and results of parasympathetic tests. In the 13 patients who were re-evaluated after an interval of 4 years there was no evidence of progression of the parasympathetic neuropathy. The results suggest that chronic haemodialysis treatment halts parasympathetic damage in patients with chronic renal failure.