The effects of exercise on energy substrate metabolism persist into the post-exercise recovery period. We sought to derive bicarbonate retention factors (k) to correct for carbon tracer oxidized, but retained from pulmonary excretion before, during, and after exercise. Ten men and nine women received a primed-continuous infusion of [¹³C]bicarbonate (sodium salt) under 3 different conditions: (1) before, during and 3 h after 90 min of exercise at 45% VO₂peak, (2) before, during and 3 h after 60 min of exercise at 65% VO₂peak, and (3) during a time-matched resting control trial, with breath samples collected for determination of ¹³CO₂ excretion rates. Throughout the resting control trial, k was stable and averaged 0.83 in men and women. During exercise, average k in men was 0.93 at 45% VO₂peak and 0.94 at 65% VO₂peak, and in women k was 0.91 at 45% VO₂peak and 0.92 at 65% VO₂peak, with no significant differences between intensities or sexes. After exercise at 45% VO₂peak, k returned rapidly to control values in men and women, but following exercise at 65% VO₂peak, k was significantly less than control at 30 and 60 min post-exercise in men (0.74 and 0.72, respectively, P < 0.05) and women (0.75 and 0.76, respectively, P < 0.05) with no significant post-exercise differences between men and women. We conclude that bicarbonate/CO₂ retention is transiently increased in men and women for the first hour of post-exercise recovery following endurance exercise bouts of hard but not moderate intensity.