The effect of exercise on particle deposition in rats was investigated. Twenty male rats were trained to run on a treadmill and were exposed to gallium-67 oxide (67Ca203) particles (0.1 pm activity median diffusion diameter) for 30 min while running at 30mmin-'. Twenty resting controls were exposed in the same system while confined in wire mesh cages. Ten exercising and 10 resting rats were killed 2 h and 12 days after exposure. Tissue radioactivity levels of 67Ca203 were measured and normalized for differences in exposure concentration and body weight. Significantly (P < 0.05) more 67Ca was deposited in the nasal passages (20 vs 5 nCi) and in the trachea and mainstem bronchi (0.05 YS 0.03 nCi) of exercising rats than in resting rats. There were no significant differences between the exercising and resting rats in amounts of 67Ca in the lung lobes at 2 h after exposure. Using assumed minute volumes, exercising rats had a significantly (P < 0.05) lower lung deposition efficiency, expressed as percentage of estimated inhaled material, than did resting rats (3 vs 10%). The results suggest that exercising rats inhale more ultrafine particles, but deposit a smaller fraction of them in their lungs. The result is a similar lung burden of 0.1 pm particles in resting and exercising animals.