The acute pulmonary response to three hours' exposure to 3 ppm formaldehyde (HCHO) during intermittent exercise was evaluated in nine healthy nonsmokers. The protocol consisted of clean air on the first day and HCHO on the second day with a 24-hour follow-up on the third day. Pulmonary function, nonspecific airway reactivity, and symptoms were assessed daily. Thirty minutes of HCHO exposure resulted in a 2% decrease in forced expiratory volume at one second (P < .05) and a 7% decrease in forced midexpiratory flow rate25%–75%(P < .01): however, these effects were no longer present between 60 and 180 minutes. There was also a significant increase in odor (P < .02), nose or throat irritation (P < .01), and eye irritation (P < .01) with exposure. No changes in pulmonary function or airway reactivity were observed 24 hours after exposure. Acute exposure to 3 ppm HCHO produced small, transient decreases in pulmonary function and mild to moderate eye and upper respiratory tract irritation.