A.J. Clark, FRS (1885–1941) held the established Chair of Pharmacology from 1918 to 1926. After qualifying in medicine, and serving as a field medical officer throughout the First World War, Clark had been appointed Professor of Pharmacology at the University of Cape Town where he remained until accepting the Chair of Pharmacology at UCL in 1920. His influence on the subject was profound. The distinguished physiologist and Nobel laureate A.V. Hill (Archibald Hill) had begun the quantitative study of the action of agonists on an isolated tissue (frog skeletal muscle) some years earlier. Clark took this much further and extended it to examine the actions of antagonists. The data he gathered on the exact relationship between agonist concentration and response, and on how this changed in the presence of a competitive antagonist, were published in two classic papers in the Journal of Physiology in 1926,.  But he failed to work out a method for analysing properly the results of experiments with antagonists: that had to wait for his successor, Heinz Schild.
Nevertheless, Clark was largely responsible for the transition of pharmacology from a descriptive subject to the quantitative science that it is today - this emphasis on quantitative approaches has remained strong throughout the subsequent history of the department. Clarks book The Mode of Action of Drugs on Cells' (Williams & Wilkins, 1933) is a classic and the following quotation from it set the tone for the department for many years.
::"In the first place, there is no advantage in fitting curves by a formula unless this expresses some possible physico-chemical process, and it is undesirable to employ formulae that imply impossibilities. It is a question of finding a few systems so simple that it is possible to establish with reasonable probability the relation between the quantity of drug and the action produced."
While at UCL Clark wrote the first edition of his textbook Applied Pharmacology in 1923, a book that was to be updated by two of his successors as Head of department, first by H.O. Schild and later by H.P. Rang, and is still extant in the form of the widely used textbook Rang & Dales Pharmacology'.
In 1926 Clark followed his predecessor in moving to the University of Edinburgh.