If a series of verbal units is presented for study, and if a unit occurs two or more times in the series, the practice schedule for these multiple occurrences may be massed or distributed. In massed practice (MP) the item occupies adjacent positions; in distributed practice (DP) successive occurences are separated by at least one other unit. The spacing effect, or the MP-DP effect, refers to better recall following DP schedules than following MP schedules. Four studies discussed in this report examined the spacing effect in four quite different situations: recognition of letters, verbal discrimination, short free recall lists, and recall of MP items presented twice with an intervening interval inserted to produce forgetting. MP-DP differences were found in all studies. Of particular interest were three interactions; subjects with a low criterion of responding in the letter study lost the MP-DP effect over a 30-second delay, and subjects with a high criterion did not; a clear MP-DP effect, but no lag effect, was found only with unmixed verbal discrimination lists; and in free recall a sharp lag effect was shown for words presented three times but not for words presented twice. (Author/LL)