1. The affinities and efficacies (relative to isoprenaline) of prenalterol and corwin at beta 1- and beta 2-adrenoreceptors, have been determined in isolated cardiac and vascular tissues respectively. 2. Prenalterol and corwin have similar affinities for cardiac beta 1-adrenoreceptors. The affinity of prenalterol for beta 2-adrenoreceptors is approximately 10 times lower than for beta 1-adrenoreceptors; that for corwin is approximately 100 times lower than for beta 1-adrenoreceptors. 3. The efficacies of prenalterol and corwin, relative to isoprenaline, at beta 1 and beta 2-adrenoreceptors, are similar. 4. The greater selectivity of corwin compared with prenalterol, as an agonist at beta 1-adrenoreceptors, is a reflection of its lower affinity for beta 2-adrenoreceptors.