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A patient sitting still might have a heart rate of, say, 75 beats per minute. However, if one were to plot the heart rate over time, it would not remain exactly at 75, but would fluctuate around some average. These fluctuations might just look like random zigzags, but using the mathematical technique of spectral analysis, it becomes possible to coax useful data about the condition of the patient from these seemingly random fluctuations. Spectral analysis views any plotted line as a complicated waveform, and, as such, may be thought of as the sum of a great many regular sine waves of different frequencies. Some frequencies contribute more to this complex mathematical wave and thus are said to be of higher ''power''; other wavelengths contribute little or nothing. The graph of different wavelengths against the power at each wavelength is known as the power spectrum. This analytical technique has been used to investigate the heart rates of patients with diabetes. Diabetic patients quite commonly develop problems that affect the nerves of the autonomic nervous system. However, there is no universally agreed upon method for detecting such neuropathy in its early stages, and some proposed methods are quite elaborate. Some methods, such as exercise testing, require significant patient cooperation. Recording the heart rate is simple and straightforward. The heart rates were recorded both lying down and sitting up for 15 patients with known diabetic neuropathy. Power spectra obtained in the supine position, presumed to reflect activity of the parasympathetic nervous system, were found to correlate with several more traditional measures of autonomic function, including the expiratory-inspiratory ratio, the rate change on deep respiration, the standard deviation of the heart rate, and the Valsalva maneuver. The change in power spectra upon sitting up, presumed to reflect sympathetic nervous activity, was a modest predictor of the change in blood pressure upon postural change, but was not a good predictor of response to isometric exercise or the response to cold stimuli. The results indicate that the power spectrum of the heart rate, which is becoming easier to obtain as computer technology becomes smaller, cheaper, and more readily available, may well become a useful routine measure of autonomic function. (Consumer Summary produced by Reliance Medical Information, Inc.)