The robustness of the findings of the BSR effects of abused substances indicates that almost any technique used to measure these effects will be useful. The simplest procedure, however, of selecting a single intensity of stimulation and determining the effects of drugs on the rate of response for that selected intensity is fraught with difficulty in interpretation. More than 25 years ago the interpretation of changes in response rate as a reflection of changes in the reward value of the stimulation was challenged (Hodos and Valenstein 1962). Today it is rare to see a single stimulation intensity used in a published manuscript. However, most studies still use procedures of which rate of response is an integral part. Because animals will press a lever more than 80 times per minute and--on a continuous reinforcement schedule--receive as many as 80 stimulations a minute, conclusions about the specificity of the effects of drugs are difficult. The results of BSR experiments on mechanisms of action of abused substances clearly indicate that the reinforcing effect of most, if not all, such substances is probably the result of activation of a reward system that originates in the cell bodies of the ventral tegmental area and courses rostrally to the limbic and frontal projection sites of the mesocortical system. Thus, we believe, the technique has clearly fulfilled its promise as a "window on the brain" (Olds 1977).