SUMMARY1. The β‐adrenoceptor stimulants MJ9184–1 (2'‐hydroxy‐5'‐[1‐hydroxy‐2‐(2‐methyl‐1‐phenyl‐2‐propylamino)ethyl]methanesulphonanilide) and (—)‐isoprenaline have been compared for their ability to produce agonistic actions in isolated atrial and in tracheal preparations from guinea‐pigs and cats.2. The ability of the compounds to produce cardiac chronotropic actions and to reduce serotonin‐induced increases in pulmonary resistance has been assessed in anaesthetized guinea‐pigs. The findings have been compared with those obtained previously in cats.3. There are marked differences in the relative stimulant activity of the two compounds between tissues, between species and betweenin vivoandin vitrosituations.4. The results suggest that β‐adrenoceptors in cat and guinea‐pig may differ. Preparations suitable for predicting the selectivity of action of