Four components of the heart period power spectrum--ultra low frequency (less than 0.0033 Hz), very low frequency (0.0033 to less than 0.04 Hz), low frequency (0.04 to less than 0.15 Hz) and high frequency power (0.15 to 0.40 Hz)--plus total power (1.157 x 10(-5) to 0.4 Hz for a 24-h electrocardiographic [ECG] recording) all predict mortality after myocardial infarction. To determine the time course and magnitude of recovery for these measures of heart period variability, 68 patients in the Cardiac Arrhythmia Pilot Study (CAPS) placebo group who had 24-h ECG recordings at baseline, 3, 6 and 12 months after myocardial infarction were studied. The 24-h power spectral density was computed with use of fast Fourier transforms and divided into the four components listed previously. The values for the five frequency domain measures of heart period variability in the CAPS patients were similar to those found in 715 patients who participated in the Multicenter Post Infarction Program (MPIP), indicating that the CAPS sample is generally representative of postinfarction patients with respect to these measures. The values for the five measures were one third to one half of those found in 95 normal persons of similar age and gender. There was a substantial increase in all measures of heart period variability between the baseline 24-h ECG recording and the 3-month recording (p less than 0.001). Between 3 and 12 months, the values were quite stable for the group as a whole, as well as for individual patients (intraclass correlation coefficients greater than or equal to 0.66).(ABSTRACT TRUNCATED AT 250 WORDS)