Maximal oxygen intake has been proposed as a standard for cardiovascular fitness. Measurement of maximal heart rate, blood pressure, oxygen consumption, and exercise duration can assess the individuals' functional aerobic capacity or reserve resulting from cardiac disease. The purpose of the present study is to evaluate the physiologic response to maximal treadmill exercise. The Chinese Air Force protocol (CAF protocol - a constant speed of 5.6 kmph with a 5% increase in grade every 3 min) was used in 202 healthy men of 30 to 57 years of age. Their mean maximal heart rate and maximal oxygen consumption are 181 b.m-1 and 40.4 ml.kg-1.m-1, respectively. The study revealed that aging bears a prominent influence on maximal heart rate and maximal diastolic pressure but not on systolic pressure. There are two major influences on max .Vo2, namely, age and habitual physical activity level. Activity had a more significant effect than the age factor. Maximal oxygen consumption was lower in sedentary than in physically active persons and diminished with age. It had a high correlation with duration of exercise by this standardized protocol. Accordingly by regression equations, average normal values from healthy persons could be predicted from activity status and age, and values expected on testing could be estimated from the duration of the exercise.