In previous studies comparing circulatory responses to exercise in men and women, the habitual physical activity of the groups was not documented. Thus, it is possible that sex differences observed were partly a function of differences in level of physical condition. The purpose of this study was to compare central circulatory responses to submaximal bicycle ergometer exercise in equally trained men and women. Cardiac output (Q), stroke volume (SV), heart rate (HR), and arteriovenous oxygen content difference [(a-v)O2 diff] were determined at approximately 30%, 50%, 70%, and 90% VO2max in 18 male and 18 female trained young adults. Q was determined by the CO2 -rebreathing method. The men and women had similar training backgrounds and nonsignificantly different mean VO2max in ml . kg FFW-1 . min-1 (62.3 and 60.3, respectively). Mean differences between men and women in Q (0.44 l . min-1), HR (23 bts . min-1), and (a-v)O2 diff at 1.5 l . min-1 and in heart rate at various percentages of VO2max (2-4 bts . min-1) were smaller than in previous research. Smaller sex differences in various VO2max expressions in the present study suggest that there was a difference between males and females in habitual physical activity in earlier research. It is concluded that a portion of previously reported sex differences in certain circulatory responses to submaximal exercise was a consequence of different levels of physical condition of the male and female subjects. The magnitude of gender-related differences in circulatory responses to submaximal exercise appears to be smaller than previously thought.