Several studies have recently established that chlorpromazine will affect the acquisition and extinction of a conditioned avoidance response in rats (ADER and CLINK 1957; MILLER et al. 1957;1957a). ADER and CLI~X (1957) used saline, 1.5 mgm./kgm., or 3.0 mgm./kgm, of chlorpromazine and found an increase in the number of trials required to learn an avoidance response as the dose level increased. Ss given 3.0 mgm./kgm, prior to extinction were found to extinguish the response most rapidly followed by the 1.5 mgm./kgm, group, with the placebo group being the most resistant to extinction. MILLER et al. (1957) found more rapid extinction of an avoidance response as the dose level of chlorpromazine increased up to 1.25 mgm./ kgm. They also found, by the use of a phenobarbital control group, that the more rapid rate of extinction was not a function of sedational effects or motor impairment brought about by the chlorpromazine. The square root of the number of avoidance responses during extinction was found to be linearly related to the log of the drug dosage.
In a subsequent study these same workers (MILLEI~ et al., 1957a) tested one group of rats for extinction of an avoidance response shortly after injecting chlorpromazine. A second group was injected at the same time but not tested until five days later. Two corresponding saline groups were also employed. The group which was given the drug and tested shortly thereafter was found to extinguish the response most rapidly. No differences were found among the other three groups.
These studies employed the rat as the experimental animal and avoidance learning as the criterion for assessment of the effects of chlorpromazine. Since the results of these studies are thought to have