Recently, we found that the reactivity of ambulatory blood pressure (BP) to everyday physical activities is highest in the morning. All participants in that study slept normally at night and freely chose their activity levels, which did not allow a separation of any circadian influence on the BP response from the effects of sleep per se. Therefore, the aims of the present study were to investigate whether there is circadian variation in the BP response to a controlled bout of exercise, and whether or not such variation is explained by the residual masking effects of nocturnal sleep. Following 4 h of nocturnal sleep, six normotensive males exercised on a cycle ergometer at 04:00, 06:00, 08:00, and 10:00 h. On a separate day, participants also slept for 4 h in the afternoon and then exercised at 16:00, 18:00, 20:00, and 22:00 h. Mean arterial BP, cardiac output (CO), heart rate (HR), and total peripheral resistance (TPR) were measured for 5 min before and 5, 10, 15, and 20 min after each exercise bout. Post-exercise data were subtracted from pre-exercise baselines and analyzed using general linear modeling with repeated measures. Fifteen min after exercise at 04:00 h, mean arterial BP was 8-14 mm Hg higher (p<0.05) than it was after the corresponding post-exercise time at the other clock-hour trials, including the 16:00 h bout that immediately followed daytime sleep. Significantly (p<0.05) greater responses of TPR and HR were also found after the 04:00 h exercise bout. We conclude that mean arterial BP shows highest reactivity to a controlled bout of exercise when performed in the morning. This phenomenon cannot be attributed simply to the residual effects of sleep, as it was not observed when participants exercised after a period of daytime sleep.