The effects of tokens as reinforcers on IQ test performance was investigated in 45 preschool Head Start children. There were 63 children assessed using the Slosson Intelligence Test for Children (SIT), and based upon these scores, were divided into three IQ groups: low, average and high. There were 15 children randomly selected from each group and within each of these groups, subjects were randomly assigned to one of three conditions: Control (C) , Pretest experimental (E1), and no pretest experimental (E 2). The C and E1 groups were administered the Peabody Picture Vocabulary Test (PPVT), Form A, according to standardized procedures. Three weeks later all groups were assessed using the PPVT, Form B, with a token being contingent on correct responses. Three weeks later all children were assessed with a standardized administration of the PPVT, Form A. Results showed that tokens given contingent upon each correct response increased the IQ scores for the initially low IQ subjects, but had no significant effect on the scores of the average and high IQ subjects. The increase in the IQ scores of the low IQ subjects was stable over time. The effectiveness of the reinforcer was empirically demonstrated.