The efficacy of nebulized glycopyrrolate compared with metaproterenol was evaluated in 46 patients with acute asthma. In a double-blinded, randomized fashion, patients received, as sole therapy, either 2 mg of glycopyrrolate or 15 mg of metaproterenol every 2 h over a 6-h study period. Of the 35 patients completing the study, analysis of variance demonstrated no difference in percentage of change in FEV1 between glycopyrrolate and metaproterenol. Two hours after the initial dose, there was a 30 percent increase in FEV1 for glycopyrrolate compared with a 25 percent increase for metaproterenol (p greater than 0.05, NS). In contrast to the comparable bronchodilator activity, the side effects profile of the two agents were markedly dissimilar. Not only were subjective complaints of tremor, palpitations, and paresthesias increased for metaproterenol, but the heart rate response was significantly elevated (p less than 0.05) compared with glycopyrrolate. Based on these data, administration of the aerosolized anticholinergic agent, glycopyrrolate, is a reasonable therapeutic alternative for acute asthma.