This study explored the degree of verbal short-term memory deficit among individuals with Down syndrome and Williams syndrome, and the extent to which any such impairment could be accounted for by a relative slowing of rehearsal and output processes. Measures of serial recall and detailed assessments of speeded articulation for short and long words were assessed among these two populations, and controls. Both clinical groups showed an impairment in serial recall. Among individuals with Williams syndrome this deficit could be explained in terms of a general slowing in speech rate. However, although aspects of speeded articulation were delayed among individuals with Down syndrome, this could not account for the extent of impairment in their verbal short-term memory performance. The implications of these findings for the source of impaired verbal short-term memory associated with Down syndrome, and for the word length effect at different levels of development, are discussed.