This study examined 4- and 6-month-olds' responses to static or dynamic stimuli using behavioral and heart-rate-defined measures of attention. Infants looked longest to dynamic stimuli with an audio track and least to a static stimulus that was mute. Overall, look duration declined with age to the different stimuli. The amount of time spent in sustained attention at 4 months, but not at 6 months, was related to stimulus discrimination. These results indicate that the decline in look duration typically observed during the middle of the 1st year for static stimuli does generalize to dynamic stimuli. The results further suggest that the amount of time spent in sustained attention during habituation may be an important indicator of processing in younger infants. (Contains 2 figures, 2 tables, and 2 footnotes.)