RET is based upon the belief that people are disturbed because of their cognitions (ideas, beliefs) about events, not because of the events themselves. Internal-external locus of control is a measure of a person's expectancy that the rewards they receive are contingent upon their behavior (internal control) rather than upon external forces. It was hypothesized that therapeutic change brought about by RET could be measured by the SLT. Subjects were 22 inmates in the drug and alcohol abuse program at a correctional institution. Seventeen subjects, the control group, entered the program. The study sought to determine if the subjects would significantly change their measured level of rationality and internal-external locus of control as a result of 4 months of closed, time-fixed RET. Results indicated a treatment effect; experimental subjects scored lower (more reational) on the posttest Personal Beliefs Inventory. There was no significant change in the posttest internal-external scores for the experimental group however, showing an indication that the internal-external level is fairly resistant to change with these subjects and treatments. Twenty-five references are noted and appendixes contain study data.