To study the cardioselectivity of xamoterol, eight asthmatic patients took part in a randomised, double-blind, cross-over study, in which xamoterol or saline were infused, followed by four increasing doses of terbutaline i.v. Circulatory studies showed a significant increase of systolic blood pressure after xamoterol 0.1 mg/kg compared to saline (P less than 0.05), and heart rate tended to increase. Diastolic blood pressure did not show any significant changes after the different treatments. Skeletal muscle tremor measurements with terbutaline stimulation did not show any differences after pre-treatment with either xamoterol or saline. Mean values of FEV1 did not reveal any significant difference before or after terbutaline stimulation between xamoterol and saline pre-treatments. However, in one patient, FEV decreased 60% after xamoterol, an effect which was reversed by terbutaline. Xamoterol did not have any effect on beta-adrenoceptor mediated skeletal muscle tremor and no significant effect on beta-adrenoceptor mediated bronchodilation in doses which gave a significant increase of systolic blood pressure. Thus, xamoterol was shown to be a selective beta 1-adrenoceptor agonist in man.