[en] Since the end of the sixties, number of studies have put in obviousness that the short-term memory span of mentally retarded subjects remains largely below the level expected regarding their mental age and their chronological age. Mackenzie and Hulme (1987) and Hulme and Mackenzie (1992), for example, showed that verbal short-term memory abilities of mentally retarded subjects do not develop at the same rate than other cognitives aspects. These results provide important information on the development of memory span in those subjects but do not provide information on the functioning of the different components of their verbal short-term memory. We conducted several experiments in order to supply with this deficit and to describe the functioning of the verbal short-term memory in a group of Down's syndrome subjects. Our data put in obviousness the similarity of verbal short-term memory's functioning in Down's syndrome subjects and in normal children (phonological similarity effect and word length effect). The absence of a significant correlation between articulation rate and memory span in Down's syndrome people (like in less than 6 years old normal children, see Gathercole et al., 1993, 1994) leads us to discuss a central point of the working memory model (Baddeley, 1986).