In four experiments, massed-practice (MP) or distributed-practice (DP) items were isolated from other items in a list presented either visually or auditorially for free recall. For three independent groups, the first, second, or both presentations of an item were isolated. Locus of isolation had a marginal influence on recall. When MP items were isolated, they were recalled better than DP items. Isolating the DP items enhanced the MP·DP effect. The results are congruent with the idea that the MP-DP effect in free recall results, in part, from differential rehearsal.Distributed practice (DP) on verbal events presented for free recall results in higher levels of recall than does massed practice (MP). In addition, a frequent finding has been that the recall of DP items increases as the lag (separation) between repetitions increases. Support for these two assertions comes from several experiments that differed widely along such parameters as rate of presentation, frequency of item presentation, and kind of to-be-remembered items (e.g., D 'Agostino & DeRemer, 1972;Elmes, Greener, & Wilkinson, 1972;Melton, 1970;Underwood, 1970).The two most viable explanations of the MP-DP effect (Elmes et al, 1972) seem to be the differential-rehearsal (e.g., Underwood, 1970) and the variable-encoding (e.g., Melton, 1970) hypotheses. According to the differential-rehearsal notion, the MP·DP effect occurs because Ss pay less attention to the second MP presentation than they do to the second DP presentation. The variable-encoding hypothesis asserts that more new information is encoded on successive presentations in DP than in MP and that this variable encoding increases as lag increases.Since both of these hypotheses refer to aspects of the storage process, it isnot surprising that current evidence indicates that both differential rehearsal and variable encoding contribute to the MP·DP effect (D'Agostino & DeRemer, 1972): MP-DP differences in recall result from differential rehearsal, whereas improved recall with increases in lag results from variable encoding.The present experiments are addressed to the differential-rehearsal account of the better recall observed under DP than under MP.Both Rundus (1971) and Shaughnessy. Zimmerman, and Underwood (1972) report that Ss rehearse MP items less than they rehearse DP items. It follows, therefore, that increased rehearsal of MP items should attenuate the MP-DP effect. Conversely, increased rehearsal of DP items should *This research was supported by a R. E. Lee Research Grant. Portions of these data were presented by the second author at the meetings of the Virginia Academy of Science, Lexington, May 1972. Requests for reprints should be sent to David G. Elmes, Department of Psychology. Washington and Lee University. Lexington. Virginia 24450. 77 enhance the MP·DP effect. Differential rehearsal was effected in the present studies by isolating the appropriate MP or DP items from the remaining items in the list. The assumption behind the use of isolation as a rehearsal variable is that the is...