This study aimed at investigating long-term memory functioning in Down's syndrome subjects (DS) as compared to individuals with mental retardation of different etiology (MR) and mental-age matched normal children (MA). For this purpose, tests of verbal and visuo-perceptual explicit memory and a verbal repetition priming task were administered to 15 DS, 15 MR and 30 MA subjects. Our results document comparable verbal priming in the three groups. As for explicit memory, normal children performed better than MR individuals, and these, in turn, better than DS subjects. Compared to MR subjects, DS subjects were particularly deficient in organizing verbal material according to its categorical structure and in actively retrieving stored information. These results support a view positing heterogeneity of neuropsychological deficits across distinct etiology MR groups.