To determine whether a food's acceptability changes with time of day and degree to which a food is appropriate for a given mealtime, preferences were obtained both in the morning and the afternoon for a set of foods which included items appropriate "for breakfast", "for dinner", or for either mealtime. A second purpose of the study was to determine whether such cultural rules regarding food appropriateness had been acquired by preschool children and whether the pattern of shifting acceptability with time of day predicted for adult subjects would also appear in the preference data of the children. Participants included 25 adults and 30 three- and four-year-old children. All subjects were seen twice, once in the morning and once in the late afternoon for two identical sessions. Half of the subjects in each age group were seen first in the morning, half were seen first in the afternoon. In these sessions food preferences for the set of foods varying in mealtime appropriateness were assessed and the foods were categorized as "for breakfast" of "for dinner". The preference data of both the adults and the children showed significant preference shifts with time of day, with breakfast items more preferred in the morning than the afternoon and dinner items more preferred in the morning than the afternoon. These shifts were larger for adults than for children but not significantly so.