The purpose of this study was to investigate effects of PM1 (particulate matter with aerodynamic diameter 0.02-2 μm) inhalation on exercise performance in healthy subjects. Inhalation of internal combustion-derived PM is associated with adverse effects to the pulmonary and muscle microcirculation. No data are available concerning air pollution and exercise performance. Fifteen healthy college-aged males performed 4 maximal effort 6-min cycle ergometer trials while breathing low or high PM1 to achieve maximal work accumulation (kJ). Low PM1 inhalation trials 1 and 2 were separated by 3 days; then after a 7 day washout, trials 3 and 4 (separated by 3 days) were done while breathing high PM1 generated from a gasoline engine; CO was kept below 10 ppm. Lung function was done after trial 1 to verify nonasthmatic status. Lung function was normal before and after low PM1 exercise. PM1 number counts were not different between high PM1 trials (336,730 ± 149,206 and 396,200 ± 82,564 for trial 3 and 4, respectively) and were different from low PM1 trial number counts (2,260 ± 500) (P < 0.0001). Mean heart rate was not different between trials (189 ± 6.0, 188 ± 7.6, 188 ± 7.6, 187 ± 7.4, for low and high PM1 trials; respectively). Work accumulated was not different between low PM1 trials (96.1 ± 9.38 versus 96.6 ± 10.83 kJ) and the first high PM1 trial (trial 3, 96.8 ± 10.65 kJ). Work accumulated in the second high PM1 trial 4, 91.3 ± 10.04 kJ) was less than in low PM1 trials 1 and 2, and high PM1 trial 3 (P = 0.004, P = 0.003, P = 0.0008; respectively). Acute inhalation of high (PM1) typical of many urban environments could impair exercise performance.