A study was carried out to determine the caffeine concentration in milk of lactating women ingesting known amounts of caffeine and to determine serum concentration of caffeine of their infants. 9 lactating women and their infants were studied for 9 days. For the first 5 days, the women ingested 750 mg of caffeine daily, and for the next 4 days they ingested no caffeine and avoided coffee and other caffeine-containing products. The concentration of caffeine in samples of milk obtained by pooling aliquots from each feeding on day 5 averaged 4.3 micrograms/ml for 7 mothers who were 11-22 days after delivery on the 1st day of study. Values ranged from nondetectable (less than 0.25 micrograms/ml) to 15.7 micrograms/ml. No caffeine was detected in milk samples obtained on day 9. The mean concentration of caffeine in sera of the infants on day 5 was 1.4 micrograms/ml (range: nondetectable to 2.8 micrograms/ml). Caffeine was still detectable in 2 infants' sera obtained on day 9. Concentrations had decreased only slightly since day 5. 2 older infants and their mothers were studied as described except that on days 5 and 9 samples of milk were obtained from each feeding and analyzed individually. The average caffeine concentrations in milk consumed on day 5 were 13.4 and 28.6 micrograms/ml, respectively. The concentrations of caffeine in infants' sera on day 5 were less than 0.25 and 3.2 micrograms/ml. On day 9 milk and serum caffeine levels were below the limits of detectability.