Abstract Structural modification of 1-(3,4-dihydroxyphenyl)-2-amino-ethanol, by adding a methyl or ethyl group at the carbon alpha to the nitrogen or by adding various alkyl groups in the nitrogen center, changes sympathomimetic activity. A rank order evaluation of these changes indicates that the β-receptor population can be divided into two distinct groups, a β-1 type (found in the heart, adipose tissue and small intestine) and a β-2 type (found in the uterus, diaphragm, bronchioles and vascular bed), as described in this and previous publications from our laboratory. A review of recent published research from other laboratories suggest that similar results also may be obtained with β-receptor antagonists.