The purposes of this study were to 1) compare the patterns of responses for plasma ammonia concentration ([NH3]) during moderate- vs. heavy-intensity cycle ergometry, and 2) examine the relationship between the V O2 slow component (V O 2SC) and plasma [NH3]. Thirteen healthy, untrained men (mean +/- SEM age = 24.8 +/- 0.6 years) performed a total of eight constant power output exercises (7 minutes in duration) at two different intensities (moderate, 60% gas exchange threshold [GET] = 60% of the gas exchange threshold; and heavy, Delta 50% = 50% of the difference between GET and V O2 max). Blood was collected from an antecubital vein before the exercise, during the last 3 minutes of the 6-minute warm-up, and during each minute of the 7-minute constant power output workbout. The time course of changes in plasma [NH3] and V O2 during the two constant power output exercise intensities were assessed separately using 2 (intensity) x 7 (time) repeated-measures analyses of variance. For 60% GET, there were no significant differences in the mean normalized plasma [NH3] during the 7-minute workbout. For Delta 50%, there was a significant increase in the mean normalized plasma [NH3] during the 7-minute workbout. These findings suggest a potential relationship between exercise-induced hyperammonemia and the V O 2SC during heavy-intensity exercise.