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.