The level of physical activity of 3- and 4-year-old children was assessed in alternative physical locations by month and time of day and by age, gender, and ethnicity. Physical activity was assessed by observation with the Children's Activity Rating Scale (CARS) for up to 12 hours from 7:00 am to 7:00 pm. A sample of 191 three- and four-year-old children was observed for up to four times in the course of a year. The sample was tri-ethnic. Boys were significantly more active than girls. Activity was consistently higher outside than inside. There were significant differences in the amount of time children in this age group spent inside versus outside by time of year; the activity levels of boys and girls differed by time of year, particularly when outside. A model including gender, month, and location terms accounted for 75% of the variance in physical activity. These data further documented gender differences in physical activity among very young children using measures not subject to self-report biases but did not explain or clarify the gender differences. The substantial differences by physical location and time of year deserve future attention, but more refined methods will be needed to balance data by location and important seasonal times. An inference from these results is that activity levels among young children may be increased by encouraging them to spend more time outdoors.