The relationship between cardiosympathetic neuropathy and cardioparasympathetic neuropathy was investigated in 103 patients with non-insulin-dependent diabetes mellitus. Cardioparasympathetic nerve function was assessed by comparing electrocardiographically the expiratory and inspiratory R-R interval ratios, during a period of deep breathing, and the coefficients of variation of the R-R intervals. Cardiosympathetic nerve function was assessed by determining the uptake of [I-123]metaiodobenzyl guanidine into the myocardium. The results indicate that there is no significant correlation between cardioparasympathetic and cardiosympathetic nerve function.