Following bronchodilator therapy in asthmatic patients, a fall in arterial oxygen tension (PaO2) has been attributed to increased perfusion of persistently underventilated alveoli. We used continuous noninvasive pulse oximetry to evaluate the extent and timing of oxygen saturation (SaO2) decrease in adults following metaproterenol inhalation for acute bronchospasm. We also examined the effect of supplemental oxygen upon these factors. Baseline and peak drop in SaO2 after completion of the first bronchodilator therapy were measured and the percent change in SaO2 was calculated. A total of 47 patient visits were studied; 10 patients received supplemental oxygen. Mean age was 38 +/- 17.9 years. Baseline was SaO2 = 94.6 +/- 2.9%, peak drop SaO2 = 91.4 +/- 3.4%, and percent change in SaO2 = -3.4 +/- 2.5%. The mean time to peak drop was 24.4 +/- 15.4 minutes in the 40 patients, with an observed drop after initial treatment. The absolute change in SaO2 from baseline was significant both with and without oxygen (P less than .05 and less than .01, respectively). The group receiving oxygen had a significantly smaller percent drop and a larger proportion of patients showing no drop in SaO2 (P less than 0.01 and less than 0.03, respectively). Clinically significant oxygen desaturation can occur within 30 minutes of inhaled bronchodilator therapy. Supplemental oxygen (2-3 L/min) helps blunt the metaproterenol-induced drop in SaO2.