We describe the daily activity patterns and the between-day and -year reliabilities of the Children's Activity Rating Scale (CARS) measured in 180 Anglo-, African-, and Mexican-American children aged 3 or 4 yr. Reliabilities were assessed at up to 11 measurement periods over 3 yr. CARS scores were measured by two trained observers (alternating 2-h shifts) up to 12 working hours. There were no significant ethnic or day of week differences in the activity levels. Males had higher mean activity levels than females and higher activity was performed in the summer and fall than the spring or winter. The all-measurements reliability for a mean of 5.34 d of observation over the 3 yr of measurement was 0.57. The reliability across all days and years of measurement was 0.73. The highest level of reliability (0.75) was found for the percent of minutes spent at CARS levels 3, 4, or 5 followed by an intraclass correlation of 0.74 for percent of minutes spent at CARS levels 4 or 5. These data support the utility of using the CARS as a measure of physical activity in younger children from different ethnic groups.