The influence of intensity and daily duration of exercise on cytochrome c concentration in the three muscle fiber types was assessed in rats that were treadmill trained for 8 wk (5 days/wk) by 1 of 19 protocols. The importance of exercise duration was determined at six running intensities (10, 20, 30, 40, 50, and 60 m/min). Daily run times resulted in a maximal adaptive change for each running intensity. The general pattern of cytochrome c change, caused by increasing daily run times, followed a first-order manner in all fiber types. The final time-independent asymptotic values were dependent on the intensity of running. In addition, as running intensity was increased, the length of daily run time required to achieve the peak adaptive response was shortened. The intensity influence was very different between fiber types. In the fast-twitch red fiber, the maximal time-independent response at each work intensity was altered by exercise intensity at only submaximal work loads (i.e., the adaptation from 19 to 29 nmol/g was not further increased at 50 and 60 m/min). In contrast, the adaptive response in the fast-twitch white fiber began at 30 m/min (approx 80% VO2max) and increased exponentially as intensity was increased (200% increase at 60 m/min). Whether this different adaptive response between fiber types is due solely to the ordered recruitment of motor units remains to be determined.