Self-perception theory predicts a decrease in preference for a rewarded activity (the "over justification effect') only when two conditions are met: Internal interest in the activity is high and the reward is perceived as more than adequate justification for performance. In order to test this prediction, it is necessary to have a baseline of internal interest that is uncontaminated by external factors. In a 2 X 3 factorial design, 60 elementary school children played with an interesting or uninteresting toy, and were given one of three levels of reward for their play. An additional 22 children provided a baseline measure of internal interest in the toys by playing with them alone and unrewarded. The predicted interaction of internal interest and reward size was found on behavioral indices of play following the reward period. Compared to the baseline, increasing reward size was associated with a decrease in preference for the interesting toy but not for the uninteresting toy.