The effects of breathing on heart rate and heart rate variability were assessed in 6-, 8-, and 10-year-old boys. Rate of breathing affected only heart rate variability, with faster breathing producing more stable heart rate. Replicating previous findings with adults, depth of breathing affected both heart rate and heart rate variability, with deep breathing producing the fastest, most variable heart rate. The absolute effects of the respiratory manipulations on heart rate variability were found to be greater for children than for adults, and there was also a developmental trend of both decreasing heart rate and variability. Implications of these findings for developmental studies using heart rate as a dependent measure are discussed.