The role of experimental psychology in the development of psychopharmacology has largely been ignored in recent historical accounts. In this article the authors attempt to redress that gap by outlining work in early experimental psychology that contributed significantly to the field. While psychiatrists focused on the therapeutic nature of drugs or their mimicry of psychopathology, experimental psychologists used psychoactive drugs as tools to study individual differences in normal behavior as well as to develop methodologies using behavior to study mechanisms of drug action. Experimental work by Kraepelin, Rivers, and Hollingworth was particularly important in establishing drug-screening protocols still used today. Research on nitrous oxide and on the effects of drug combinations is discussed to illustrate the importance of experimental psychology to psychopharmacology.