Selectivity for beta 1- and beta 2-receptors to xamoterol, prenalterol and salbutamol were tested using ICI 118 551, a specific beta 2-receptor antagonist. Measurements were made of heart rate at rest and exercise, blood pressure, forearm blood flow and finger tremor. The actions of xamoterol were similar to those previously demonstrated, and were unaffected by beta 2-blockade, indicating selectivity for the beta 1-receptor. Salbutamol was selective for the beta 2-receptor and prenalterol was active at both.