A sample of 335 five-year-old children participating in an ongoing longitudinal study was the focus of a study on the effects of emotional and behavioral challenge on cardiac activity in children with different patterns of early childhood behavior problems. The children were placed in one of three behavior problem groups (low behavior problems, risk for externalizing problems, risk for mixed externalizing/internalizing problems) based on their scores on the Child Behavior Checklist for 4-18-year-olds [Achenbach, T.M., 1991. Integrative guide for the 1991 CBCL/4-18, YSR & TRF profiles. University of Vermont Department of Psychiatry, Burlington, VT], completed by their mothers. To assess cardiac vagal regulation, resting measures of respiratory sinus arrhythmia (RSA) and RSA change (vagal withdrawal) to five emotionally and behaviorally challenging tasks were derived. In addition, heart period (HP) and heart period change (HR acceleration) was examined. Results indicated that the behavior problem groups did not differ in terms of resting measures of either RSA or HP. Analyses of the challenge tasks indicated that the children at risk for mixed problems displayed greater cardiac vagal withdrawal across the five tasks than did the other two groups of children. There was a trend for the children at risk for externalizing problems to display less vagal withdrawal than the control group. In addition, the children at risk for mixed problems displayed greater heart rate acceleration to the tasks than did the other two groups of children. Follow-up analyses indicated that the greater cardiac acceleration observed in the mixed group was largely a function of greater vagal withdrawal. These findings are discussed in terms of the emotion regulatory function of cardiac vagal regulation, and its implications for patterns of risk for behavior problems in young children.