Various applications of pA(x) measurements are discussed based on the hypothesis that drugs and drug antagonists compete for receptors according to the mass law. Examples are given illustrating the use of pA(x) measurements to identify agonists which act on the same receptors and to compare the receptors of different tissues. Tests of competitive and noncompetitive antagonism are considered in relation to the antagonisms acetylcholine-atropine, histamine-atropine and acetylcholine-cinchonidine. A new measure, pA(h), is introduced to express the activity of unsurmountable antagonists.