The experiments was designed to discover the way in which the deleterious effects of a mildly depressant drug, chlorpro-mazine, on trained responses combine with those of training level. The trained response was a water-reinforced lever-pressing response in the rat. The effect of a mild dose (2.5 mgAg) of chlorpromazlne was studied at 5 different levels of habit strength (H), with 4 rats in each H group. Results indicate very clearly that highly trained responses become less susceptible to the deleterious effects of the drug, a finding contrary to those of previous researches.