This study reports evidence of ultradian rhythms in heart rate of children during play and nap time. The assumption of this study is based on the basic rest-activity cycle (BRAC) which manifests during wakefulness and sleep as a continuous REM-NREM cycle with 60-70 min period in children. The electro-cardiogram during play and nap time was recorded. The data were subjected to spectral analysis and were examined for the existence of periodicities. "There were the approximate 40 min and 20 min rhythms during play, while the dominant 77 min rhythm can be detected during nap time. Slow rhythms during both play and nap time appear to be an example of the BRAG though these rhythms differ in period, but the fast rhythm has no clear basis. Children's play was found to be modulated by the two rhythms.