SummaryThe method of operant conditioning recommends itself as particularly adequate in the study of behavioural action of psychotropic drugs. Its advantages and possibilities in psychopharmacological analysis are discussed. As illustration, experimental results are presented, concerning two central problems : 1) behaviour as an independent variable in the study of drug action and 2) the behavioural aspects of long term pharmacological treatments. The behavioural analysis raises new problems to pharmacologists, some of which are discussed with reference to concepts and data in pharmacology.