the pathway to the quantitative analysis of behavioral manifestations was opened by the research of Pavlov and his collaborators. The basic phenomenon that they investigated intensively was the conditional reflex, conditioning being the method devised to approach this problem. Pavlov’s experiments were conducted on dogs that had undergone a simple operation consisting of exposing the internal end of the salivary duct through an opening made in the cheek. This operation made possible an accurate measurement of the salivary secretion, either by counting the drops or by collecting them in a graded container. The animal was isolated in order to exclude any foreign, external stimulation, and the experimenter observed its behavior from an adjoining room, aided by mirrors. The experiments conducted by Pavlov follow, in order of sequence.