Smoking is known to have various effects on metabolic function as well as on the respiratory function in the lung. We report here that exposure to smoke causes a rapid elevation of the level of serum angiotensin-converting enzyme in dogs in vivo. In 7 dogs, the level of serum angiotensin-converting enzyme increased from a basal level of 11.0 ± 5.0 to 13.9 ± 4.0 U/ml after 60 min of exposure (mean ± SD, p < 0.01). A similar elevation of serum angiotensin-converting enzyme level was noted in dogs that received an intravenous administration of nicotine. The possible mechanism of the change is discussed.