In an attempt to study autonomic function during the 5-minute period preceding ischemic ST segment depression (decreases ST) episodes, we selected 138 decreases ST episodes, without preceding decreases ST during the last 15 minutes before each episode, from the Holter tapes of 35 patients with multivessel coronary artery disease. For the 5-minute period preceding each decreases ST episode, we calculated the following heart rate variability (HRV) indices; the mean RR interval (RR5), the standard deviation of all RR intervals (SD Index5), the corresponding coefficient of variation (CV5), and the natural log (Ln) of the spectral components, total power at 0.000 to 0.400 Hz (TP5), low frequency power at 0.040 to 0.150 Hz (LF5), high frequency power at 0.150 to 0.400 Hz (HF5), and the ratio of the low to high frequency power (LF5/HF5). As HRV indices of the 24-hour period, we calculated the respective RR, SD Index, CV, LnTP, LnLF, LnHF, and Ln LF/HF. RR5, SD Index5, CV5, and Ln TP5 were all significantly lower than RR (t = -5.343, p = 3.7 x 10(-7)), SD Index (t = -19.091, p = 1.99 x 10(-40)), CV (t = -15.780, p = 1.28 x 10(-32)), and LnTP (t = -3.210, p = 0.0016), respectively. LnHF5 was inversely correlated with the magnitude of the decreases ST; r = -0.174, P < 0.05, and CV5 was inversely correlated with the natural log (Ln) of the ischemic event duration; r = -0.183, P < 0.05. Analogous results were obtained for both the painful and silent decreases ST episodes. It is concluded that HRV is decreased during the 5-minute period preceding decreases ST episodes, and is inversely related with the magnitude and the duration of the *ST.