The purpose of this study was to investigate cardiovascular [heart rate (HR), blood pressures, rate-pressure product (RPP)], physiological (oxygen uptake), and metabolic (blood plasma lactic acid concentration) responses during symptom-limited graded exercise testing (SL-GXT) during early morning and late afternoon hours in an unmedicated, regularly exercising, coronary artery disease population. Subjects, males ages 41-71 yr (n = 11), underwent SL-GXT via cycle ergometry at 8:30 A.M. and 4:30 P.M. HR, systolic blood pressure, diastolic blood pressure, and RPP were evaluated at rest and at submaximal and peak work. Oxygen uptake and rate of perceived exertion were also determined at submaximal and peak power output, with blood plasma lactic acid concentration measured at peak work. Data, analyzed by the paired t test, failed to demonstrate any significant variation in HR, systolic blood pressure, diastolic blood pressure, RPP, or blood plasma lactic acid responses during exercise at the two points used. Oxygen uptake did not significantly change at any submaximal intensity. Thus SL-GXT results obtained at early morning (8:30 A.M.) and late afternoon (4:30 P.M.) are a reproducible, reliable means of prescribing exercise HR and work intensities.