Memory for serial position in four-item lists of words or abstract designs was tested at retention intervals of 5-25 seconds in two subject groups, aged 25-35 and 60-75 years; accuracy and choice reaction times (RT) were recorded. Increasing the retention interval transformed the serial position curve from recency to primacy but produced no overall reduction in memory performance in terms of accuracy. RTs varied as a function of both serial position and retention interval. Under all conditions the memory of older subjects was less accurate and they exhibited longer RTs compared to young subjects, but the age differences did not interact with list position or retention interval for either response indicator. The results suggest that age differences in memory can be explained by a single factor of mental speed limiting encoding efficiency and affecting decision times.