The Cognitive Evaluation Theory explains the outcomes of studies employing Deci's paradigm, but only when used post hoc. A basic assumption is that extrinsic rewards always increase intrinsic motivation for nonhuman subjects. Deci's paradigm was modified for use with 22 rats to test this assumption. Running in an exercise wheel was the intrinsically motivated activity studied. ANCOVA revealed that external rewards increased intrinsic interest on the first day following the cessation of reinforcement (F = 8.32), but on two subsequent days and again a week later, no significant differences between the reward and control groups were evident (F = .29; F = .33; F = 3.70). The assumption was not supported. It was demonstrated that repeated posttest measures are necessary to avoid basing conclusions upon one point along the extinction continuum.