This study was designed to examine the effects of age and time of day on work rates during prolonged, self-paced exercise. Eight young (19-25 years of age) and eight old (48-62 years of age) endurance athletes volunteered for the study. At two times of day (07:00 and 17:00 h), subjects were asked to pedal on a Monark cycle ergometer (Varberg, Sweden) at a self-chosen exercise intensity that they believed they could sustain for exactly 80 min. This self-chosen work rate rectal temperature, skin temperature (chest, arm, and lower leg), oxygen consumption (V̇O2), expired carbon dioxide (V̇CO2), minute ventilation (VE), heart rate, and perceived exertion (RPE) were recorded every 10 min during the exercise. Preexercise resting measures of rectal temperature, V̇O2, and V̇E were less affected by time of day in the older group than were those in the young subjects (p > 0.05). In the morning, rectal temperature was 0.3°C higher in the older subjects than in the young adults. Diurnal variation in mean work rate over the 80-min exercise period was not evident in the old group (p > 0.10) but amounted to 10 W in the young group (p > 0.05). Older subjects chose work rates 5.4 W lower than did the young subjects in the morning test session (p > 0.10). In the afternoon, age differences in work rate amounted to 14.3 W (p > 0.05). For all subjects, work rates remained relatively constant throughout the exercise period in the morning. In the afternoon, subjects chose high work rates within the first 40 min of exercise, after which work rate decreased sharply to values similar to those recorded in the morning (p > 0.01). These changes were mirrored closely by changes in (V̇O2) and V̇CO2. Perceived exertion increased linearly throughout exercise, irrespective of age or time of day. These results suggest that, in young adults, the mean work rate over 80 min of exercise is higher in the afternoon than in the morning, although the work rate decreased sharply toward the end of afternoon exercise. In agreement with studies reporting age-related increases in "morningness," age differences in work rate appeared to be least when exercise was performed in the morning.