In the first National Health and Nutrition Examination Survey (NHANES 1), the epidemiology of resting pulse rate was examined in preschool children. Among 2800 children aged 1 to 5 years, pulse rate was higher in younger than in older children, and in girls than in boys. Only at ages 4 and 5 was pulse rate lower in blacks than in whites. Of numerous other variables, only height and Southern geographic region showed independent associations with pulse rate. In a subgroup of 1056 children whose mothers were also examined, the child's pulse rate was significantly correlated with the mother's rate at ages 2, 3, and 5. Previously reported negative correlations with mother's blood pressure were not seen. Boys with hypertensive mothers had lower age-adjusted pulse rates, but girls had higher pulse rates than children of normotensive mothers. Further research is needed to determine the mechanisms of these epidemiologic patterns and their relationship to the risk of hypertension and heart disease in later life.