Eleven mother-infant pairs were studied to determine the effect of maternal caffeine ingestion (500 mg/day) on heart rate and sleep time of the infants. In a balanced cross-over design, mothers ingested 5 cups of decaffeinated coffee daily during one 5-day period and 5 cups of decaffeinated coffee with added caffeine (100 mg/cup) during another 5-day period. Concentrations of caffeine in milk on the last day of the caffeine period ranged from 1.6 to 6.2 micrograms/ml and the intake of caffeine by the infants was estimated to be 0.3-1.0 mg/kg/day. Infant serum did not contain detectable amounts of caffeine on the last day of either experimental period. Performance during the caffeine and no-caffeine periods was not significantly different with respect to either 24-hour heart rate or sleep time.