Three experiments were conducted to examine the effects of providing extrinsic rewards for engaging in an activity on children's subsequent intrinsic interest in that activity. In each study, preschool children were asked to engage in an activity of initial intrinsic interest in individual experimental sessions. The children agreed to engage in this target activity under three different reward conditions. After these sessions, unobtrusive measures of the children's subsequent intrinsic interest were obtained during a series of free-play periods. In each of the studies, results indicated that asking children to engage in an activity of initial interest as a means to some ulterior end proved a consistently effective method for undermining these children's intrinsic interest in that activity. In addition, the results indicated that close adult surveillance also produced a similar decrement in subsequent intrinsic interest. Suggestions to maintain children's intrinsic motivation included: 1) systems of extrinsic reward systems should be employed only when nesessary to elicit the desired behavior pattern; and 2) when necessary, such programs should attempt to employ the least powerful rewards when required to produce the desired behavior change. (Author/KSM)