The ability to reproduce from memory a short list of verbal items immediately following presentation is known to improve over successive trials on that list, even if these trials are embedded in a sequence of trials on other lists of the same sort (Hebb, 1961). Less clear is whether this "Hebb effect" arises without the list repetition being noticed. This question has long been pondered and has recently taken on particular theoretical significance, but the available evidence is scant and inconsistent. Two experiments are described in which, in essence, a sequence of immediate reproduction trials was followed by tests that called for list recognition (Experiments 1 and 2) and/or estimates of list presentation frequency (Experiment 1). These tests provided quantitative measures of repetition awareness. Typical Hebb effects were found, but there was no evidence that the effects occurred without the subjects' being aware of the repetition; effect-size analyses indicated that both the recognition and frequency responses were more sensitive to repetition than were the reproduction responses. Therefore, not only could the recognition and frequency responses not have been made solely on the basis of how readily the test lists were reproduced, but the Hebb effect could have required an awareness of repetition.