This study examined the changes in three cardiac rhythmicities (angiotensin-renin vasomotor [ARV], Traube-Hering-Meyer [THM], and respiratory sinus arrhythmia [RSA]) during prolonged attention in preschool children. Electrocardiogram data were collected from children during resting baseline and a 5-min attention condition at ages 3, 4, and 5 years in a longitudinal study. The ARV and THM rhythmicities decreased during prolonged attention for most participants. The RSA rhythmicity did not change consistently. The results were replicated at all three ages. The ARV and THM effect sizes were large and moderate, respectively. Only the ARV decrease correlated with duration of attention.