Respiratory responses to a variety of known bronchoactive agents were compared in anesthetized rhesus (Macaca mulatta) and cynomolgus (Macaca fascicularis) monkeys. Administration of 0.1 to 1.0% histamine aerosols resulted in an increase in airway resistance of 50 to 200% and a decrease in lung compliance of 30 to 80%. Aerosols of prostaglandin E2 (1 mg/ml), terbutaline (10 mg/ml), and isoproterenol (10 mg/ml) or iv aminophylline (up to 7.0 mg/kg) administered concomitantly with histamine produced a transient reversal of the histamine-induced changes in both species. Since the rhesus and cynomolgus monkeys responded in a comparable manner to these bronchodilator agents, the cynomolgus monkey appears to be an additional valuable model for the evaluation of potential bronchoactive compounds.