We used spectral analysis of heart rate variability during orthostatic load in diabetic patients in order to provide quantitative information about first, the alterations of vagal and beta-adrenergic nervous system influence on the heart and secondly, the relation of spectral components of heart rate variability to age. The respiration-related power, and indirect estimate of vagal cardiac control, was significantly reduced in diabetics as compared with controls in both the supine (diabetics: 0.39 (0.18-1.15) l/min2 [median and 25-75% percentiles], controls: 1.52 (0.54-3.84) l/min2, 2P less than 0.01) and the standing (diabetics: 0.27 (0.12-0.84) l/min2, controls: 2.22 (0.67-5.14) l/min2, 2P less than 0.0001) body posture. The difference between supine and standing values of mid-frequency power (delta MF power) that represents beta-adrenergically mediated heart rate variability was significantly smaller in diabetics (delta MF: 0.79 (0.18-3.84) l/min2) than in controls (delta MF: 8.07 (4.35-17.99) l/min2) (2P less than 0.0001). Power of almost all frequency components declined exponentially with age in both positions and groups studied. The decline in power of heart rate variability was more pronounced in controls than in diabetics for either posture. An age-associated significant decrease in delta MF was found in controls (Y = 33.16-0.60X, P less than 0.05, r = -0.49) as compared to diabetics (Y = 8.70-0.16X, P greater than 0.05, r = -0.24). Our results suggest a decreased vagal and sympathetic nervous system input into the heart in diabetics. The premature loosened coupling between heart rate spectral power and age suggests that diabetes mellitus accelerates the aging process. Computerized spectral analysis of heart rate variability seems to be a very sensitive tool to evaluate the influence of the autonomic nervous system on the heart.