Healthy volunteers 12 to 15 yr of age (46 boys, 13 girls) were exposed to purified air and to smoggy Los Angeles ambient air on different occasions. The studies were performed in random order approximately 2 wk apart. They included 1 h of continuous bicycle exercise (mean ventilation, 32 L/min) plus brief warm-up and cool-down periods. Symptoms and forced expiratory performance were recorded preexposure in purified air, immediately postexposure, and after 1 h recovery in purified air. Mean exposure temperature was 32 degrees C, and mean relative humidity was 45%. In ambient exposure, pollutant concentrations averaged 0.144 ppm for ozone and 153 micrograms/m3 for total suspended particulates. Group mean FEV1 decreased during ambient exposure (p less than 0.01) and only partially recovered during the following 1 h. Unlike adults studied previously, this subject group reported no significant increase in respiratory symptoms accompanying changes in FEV1. Adolescents may be less aware of early respiratory irritation by oxidants and thus more at risk from ambient exposures than are adults.