The effect of memory load on the cognitive pupillary response among 16 young adults and 16 older adults was investigated. Mean pupil dilation and reaction time were measured during a Sternberg memory-search task, which involved six levels of memory load. A classic interaction pattern was obtained in which the reaction times of the elderly participants increased more as a function of memory load than the reaction times of the young participants. In the encoding phase of the experiment, mean dilation increased with memory load. No age differences were observed here. In the search phase of the experiment, however, mean pupil dilation was considerably greater in the young than in the elderly participants. Moreover, mean dilation of the older participants was not sensitive to memory load, whereas mean dilation increased as a function of memory load in the young participants. The results suggest that the usefulness of the pupillary response as a correlate of subtle fluctuations in memory load diminishes with old age.