To examine the effect of downhill walking a form of negative work on ventilation, we studied the exercise responses of 13 healthy subjects during uphill and downhill walking on a treadmill. Each test lasted 16 rain and the peak work rate was 5.6 kph with either a positive or negative 14% grade. Throughout each test we recorded ~rO2 , VE, f~ VT, HR, systolic BP and Borg's rating of perceived exertion. At the target work rates of 5.6 kph +_ 14% grade, I?o2 and I?E were three times greater in uphill compared with downhill walking. However, in downhill walking, fwas greater comparedwith uphill walking wherein V~r approximated baseline values, reflecting rapid shallow breathing, and VT appeared to increase after reaching some critically-low level. These trends persisted when ~?o2 was held constant (p < 0.01). HR and systolic BP increased and decreased with the positive and negative grade respectively. At a constant 17o2 however, HR was significantly higher during downhill compared with horizontal walking (p < 0.01) whereas systolic BP was not significantly different (p > 0.05). We conclude that there is a significant difference in the ventilatory responses between the two types of work performed on a treadmill. Specifically, downhill walking is associated with rapid shallow breathing which may be countered by a protective feedback mechanism at critically-low levels of VT.