OBJECTIVE The purpose of this study was to determine the magnitude of changes in cardiac output (Qc), stroke volume (SV), and arterial-mixed venous oxygen difference (a-vO2 diff) during submaximal exercise following a 20-wk endurance training program, with the primary focus on identifying differences in response by race, sex, and age. METHODS The participants in this study (N = 631) were healthy and previously sedentary men (N = 277) and women (N = 354) of varying age (17-65 yr) and race (blacks, N = 217; whites, N = 414) who had completed the HERITAGE Family Study protocol. After baseline measurements, participants trained on cycle ergometers 3 d x wk(-1) for a total of 60 exercise sessions starting at the HR associated with 55% of maximal oxygen uptake (VO2max) for 30 min/session and building to the HR associated with 75% of VO2max for 50 min/session, which was maintained during the last 6 wk. HR, Qc (CO2 rebreathing), and SV (Qc/HR) were determined in duplicate at 50 W and at 60% of VO2max on two different days both before and after training. RESULTS After training, there were significant decreases in HR and Qc, and significant increases in SV and a-vO2 diff at 50 W (except for no change in a-vO2 diff in black men). The changes in HR differed by sex and age, and the changes in SV, Qc, and a-vO2 diff differed by race. Qc decreased by 0.6 L x min(-1) at 50 W for the total sample, consistent with the decrease in VO2 at this power output. At 60% of VO2max HR decreased, and SV, Q, and a-VO2 diff increased. There were small differences in response by sex (HR and SV), race (HR), and age (HR and Qc). CONCLUSIONS It is concluded that the cardiovascular systems of men and women, blacks and whites, and younger and older subjects are not limited in their ability to adapt to endurance training.