New .beta.-adrenoceptor blocking agents (4) carrying tyramine as the amino moiety were synthesized and the distribution of their 125I-tagged derivatives evaluated in rats. This distribution was compared with the distribution of various agonists and antagonists labeled with 3H and 14C and with the in vitro binding affinity of the new derivatives. A radioiodinated derivative of a cardioselective blocker, alprenolol, showed poor blood clearance and no cardiac selectivity. A derivative of another cardioselective blocker, practolol, showed a promising heart-to-blood ratio (approximately 19) and cardioselectivity with a heart-to-lung ratio of approximately 2. Additional practolol analogs (2) showed no improvement over the practolol derivative; because of the increased lipophilicity of these derivatives, blood clearance and cardioselectivity were diminished. An inverse correlation was suggested between the dissociation constant for the .beta.-adrenoceptor in the lung and the heart-to-blood and heart-to-lung values. Apparently polarity is important in the blood clearance and cardioselectivity of these .beta.-adrenoceptor derivatives.