The metabolic and pulmonary function effects were investigated in 6 non-smoking, young adults who were exposed for 2 hours (22 deg. WBGT) to: (1) Filtered air (FA), (2) 0.45 ppm ozone (DAY), and (3) Two days later to a second exposure to 0.45 ppm ozone exposure (DAY2). The subjects alternated 20-minute periods of rest and 20-minute periods of bicycle ergometer exercise at a work load predetermined to elicit a VE of 27 L/min (BTPS). Functional residual capacity (FRC) was determined pre- and post-expose. Forced vital capacity (FVC) was determined before and after exposure, as well as 5 minutes after each exercise period. Heart rate was monitored throughout the exposure, and ventilatory minute volume (VE), oxygen uptake (V02), respiratory rate (fR), and tidal volume (VT) were measured during the last 2 minutes of each exercise period. There were no changes in any variable consequent to FA exposure. Both ozone exposures induced significant (P<0.05) decrements in FVC, FEV-1, FEV-3, FEF 25-75%, and TLC. The decrements following the DAY 2 ozone exposure averaged 7.2% greater than those following the DAY 1 exposure.