Oral metaproterenol was administered daily for 3 months to asthmatic children. The study was designed to determine whether chronic tolerance developed to this drug. Initial and final crossovers on the first 2 and the last 2 days of the investigation to a test dose of metaproterenol and placebo failed to show tolerance as indicated by improvement in lung volumes and dynamic mechanics of breathing after metaproterenol. Forced expiratory volume in 1 sec was the test most consistent in demonstrating bronchodilation; the action of metaproterenol. Forced expiratory volume in 1 sec was the test most consistent in demonstrating bronchodilation; the action of metaproterenol appeared to last at least 5 hours as measured by this test. T of distribution of ventilation, e.g., single- and multiple-breath nitrogen washout tests, were not consistently altered by metaproterenol. These tests did not appear to be sufficiently sensitive to detect improvement even when airway resistance decreased and forced expiratory volume in 1 sec increased toward the normal range.