The relationships between frequency of rumination in a boy with profound mental retardation and a variety of environmental, interpersonal, and temporal variables were investigated by collecting and analyzing data during all waking hours over a 4-week period. Low levels of rumination were associated with periods of special education programming (versus nonschool hours), individual attention (versus group activities and independent play), and time spent with caretakers who like the child (versus those who like him less). The findings also revealed a mealtime effect (decreasing rumination as time elapsed following meals) and a time of day effect (increasing rumination as the day progressed). Directions for future research and possible implications for the environmental management of rumination are discussed.