SUMMARY1. Agonistic and antagonistic activities on β‐adrenoreceptors have been assessed for orciprenaline, terbutaline, soterenol and MJ7999–1 using isolated guinea‐pig atrial (β1‐receptor) and tracheal (β2‐receptor) preparations.2. As agonists MJ7999–1 is more selective for β2‐receptors than soterenol because of its increased activity in trachea, whereas terbutaline is more selective for β2‐receptors than orciprenaline because of its decreased activity in atrial preparations.3. Antagonistic activity was assessed from shifts in concentration‐effect curves to (‐)‐isoprenaline.4. MJ7999–1 and soterenol possess competitive β‐receptor blocking actions in atrial (pA2≈6.5) and tracheal (pA2≈6.0) preparations.5. Orciprenaline and terbutaline are relatively selective competitive β‐receptor antagonists, being more potent in atrial (pA2≈6.3) than in tracheal (pA2∼4.9) preparations. In atria non‐competitive antagonistic actions are apparent with these two drugs.6. In rat vas deferens preparations only (‐)‐isoprenaline and soterenol display agonistic actions on α‐receptors. None of the c