The purpose of this study was to determine whether metabolic and cardiorespiratory adaptations to exercise training are greater at the time of day of training than at another time. Twenty-seven subjects performed cycle ergometer tests in the morning (AM) and in the afternoon (PM) before and after a 6-wk period during which ten subjects trained regularly in the morning, seven subjects trained in the afternoon, and ten did not train. Training caused decreases in HR, VE, and rating of perceived exertion during submaximal exercise; a 7.7% increase (p less than 0.01) in VO2 max; and a 9.1% increase (p less than 0.01) in performance time. Adaptations (training effects) were independent of time of day of training for all variables except VO2 at the ventilatory threshold. Compared with each other, subjects who trained in the morning had relatively higher post-training thresholds in the morning, while subjects who trained in the afternoon had relatively higher values in the afternoon (p less than 0.05). This is evidence of circadian specificity in training and supports the notion of planning physical preparation to coincide with the time of day at which one's critical performance is scheduled.