The oxygen cost of treadmill running is well documented in young adult rats. However, to our knowledge there are no detailed reports concerning the oxygen cost of treadmill running in senescent rats. Young adult (4 months; N = 8) and senescent (24 months; N = 8) female Fischer-344 rats were tested for O2 cost of treadmill running. One-half of the animals tested in each age group had completed 10 wk of endurance training. Oxygen cost of treadmill running, using a flow-through system, was measured on a per minute basis from 7-10 min of each work bout and averaged. Separate work bouts were performed at 0%, 5%, and 10% grade at 15, 20, 25, and 30 m.min-1. Oxygen cost of treadmill running for trained rats for all speeds and grades were not significantly different (P = 0.77) from untrained animals; thus, data were pooled for oxygen cost of running. Results revealed significantly lower (P < 0.05) VO2 (expressed as ml.kg-1.min-1) for senescent rats vs young adult rats rest and at all comparable treadmill grades and speeds except for 5% grade, 25 m.min-1 and 10% grade, 20 m.min-1. In addition, resting VO2 was significantly lower (P < 0.05) in old rats when compared with young rats. Thus, we conclude that the oxygen cost of treadmill running is lower for 24-month-old rats than in 4-month-old rats except at higher work levels.