Exercise intensity is one of the major determinats in the exercise prescription, where THR plays an important role. This study was designed to reemphasize the usefulness of the Karvonen's formula with special regard to the anaerobic threshold (AT) in the exercise prescription to middle-aged healthy men. Fifty normal adult subjects between 30 and 59 years who were refered to the Life Planning Center for the evaluation of physical fitness were selected for the study. Symptom limited maximal treadmill stress testing after Bruce's protocol was performed for all subjects and AT was determined as the level, where the minute ventilatory volume curve during exercise revealed the first breaking point. The heart rate at AT (HR_AT) was compared to those obtained by the Karvonen's formula (k=0.6) and the other conventional method, in which the 70% and 85% of the maximal heart rate (HRmax) were determined as an optimal range for the exercise intensity. The following results were obtained: the average value of HRAT just coincided with the THR obtained with Karvonen's formula and its±1 S.D. values just fitted into the range of 70% and 85% of HRmax, respectively. THR by the Karvonen's formula varies with k values and is mostly affected by the resting heart rate (HRr) . If the k is fixed to 0.6, THR thus obtained can keep its range between 70% and 85% of HRmax under the normal range of HRr (mean±2 S. D. ; 37-116 bpm) . Moreover, THRs obtained from the Karvonen's formula (k=0.6) using the age predicted HRmax were best fitted into the ones derived from the HRmax obtained by the maximal exercise testings. Thus, THR detemined by the Karvonen's flrmula (k=0.6) can be the most suitable with regard to the anaerobic threshold and the most reliable within the wide range of normal resting heart rate.