Mere familiarization with a stimulus increases liking for it or similar stimuli ("mere exposure" effects) as well as perceptual fluency, indexed by the speed and accuracy of categorizing it or similar stimuli ("priming" effects). Candidate mechanisms proposed to explain mere exposure effects include both increased positive affect associated with greater perceptual fluency, and reduced negative affect associated with diminished apprehensiveness of novel stimuli. Although these two mechanisms are not mutually exclusive, it is difficult for behavioral measures to disentangle them, since increased liking or other indices of greater positive affect toward exposed stimuli could result from increases in positive feelings or decreases in negative feelings or both. The present study sought to clarify this issue by building on research showing a dissociation at the neural level in which the lateral orbitofrontal cortex (LOFC) is activated more by negatively valenced than by neutral or positively valenced stimuli, with the reverse effect for medial orbitofrontal cortex (MOFC). Supporting the reduced apprehensiveness hypothesis, we found lower LOFC activation to familiarized faces and objects (repetition suppression). We did not find evidence to support the positive affect hypothesis in increased activation to familiarized stimuli in MOFC or in other parts of the reward circuit that respond more to positively valenced stimuli (repetition enhancement), although enhancement effects were shown in some regions.