The need for tests to assess bronchodilatation or bronchoconstriction is of considerable importance in respiratory pharmacology. It can be approached either by measuring flow during conditions of maximal effort as discussed in the previous paper or by measurements of airway resistance under low flow conditions. Since the physiological basis of these two types of test is somewhat different they do not necessarily give the same information although in most situations there is general concordance. Each test is appropriate to different situations and the purpose of this article is to outline the main physiological factors underlying measurements of airway resistance and discuss the situations where these measurements may be more appropriate than measurements of maximum flow.