The interaction of isoprenaline with propranolol and pindolol at beta-adrenoceptors in isolated guinea pig atria, trachea and fat cells has been investigated. The affinity constants obtained revealed that in all three preparations both drugs acted as simple competitive antagonists. Although pindolol showed a somewhat greater affinity for atrial receptors than for those in the trachea neither of the antagonists showed any marked selectivity for adrenoceptors in either of these two preparations. In contrast, the affinity of fat cell receptors for either antagonist were markedly and consistently lower than those of the atrial or tracheal adrenoceptors. The theoretical implications of these results are discussed.