We have examined the immediate heart-rate response to standing to lying (S-L) in 83 male insulin-dependent diabetic subjects aged 40 +/- 11 years (mean +/-S.D.) who underwent five other cardiovascular autonomic tests. Using a specially devised scoring system, the patients were divided into 3 groups: 54 subjects without autonomic neuropathy; 17 'borderlines'; 12 with autonomic neuropathy. The results were compared with those of 50 sex and age matched controls. We evaluated: S-L1 = ratio between R-R mean before lying and R-R minimum over the first 5 beats after lying; S-L2 = ratio between R-R maximum between the 20th to 25th beat and R-R minimum over the first 5 beats after lying. In controls S-L1 was 1.23 +/- 0.098 (mean +/- S.D.), S-L2 1.56 +/- 0.2. In diabetic subjects without autonomic neuropathy S-L1 was 1.18 +/- 0.096 (p less than 0.01), S-L2 1.50 +/- 0.23. In the autonomic group S-L1 was 1.03 +/- 0.01 (p less than 0.001), S-L2 1.16 +/- 0.086 (p less than 0.001). We propose that the lowest normal and highest abnormal limits of S-L1 are 1.10 and 1.07, respectively, and that normal and highest abnormal limits of S-L2 are 1.23 and 1.41, respectively. We suggest the use of S-L1 as a pure parasympathetic test and S-L2 as a mixed but predominantly sympathetic test in the diagnosis of autonomic neuropathy.