Abstract: Malfunctions of the cardiovascular control system may be diagnosed by examination of a patient's heart rate fluctuation power spectra. Particularly indicative of cardiovascular stress are: a level below about 0.1 (beats/min.).sup.2 in the power spectrum of heart rate fluctuations at a frequency between about 0.04 and about 0.10 Hz; a marked increase to above about 10 (beats/min.).sup.2 in a peak in the heart rate fluctuation power spectrum between about 0.04 to 0.10 Hz; and a ratio of the area under a heart rate power spectrum peak at a frequency between about 0.04 and 0.10 Hz to the area under a peak in the respiratory heart rate fluctuation power spectrum centered at the mean respiratory rate about about 0.10 Hz as having an absolute value less than 2.0 for longer than or equal to about one hour or as having an absolute value greater than about 50.