Six nursing mothers ingested 113 gm of Hershey's milk chocolate containing 240 mg of theobromine. Samples of plasma, saliva, and breast milk were assayed for theobromine by high pressure liquid chromatography. Peak theobromine concentrations of 3.7 to 8.2 mg/l were found in all fluids at 2 to 3 hour after ingestion of chocolate. The disposition half-life of theobromine averaged 7.1 +/- 2.1 hours, body clearance was 65 +/- 20 ml/hour/kilogram, and the apparent volume of distribution was 0.62 +/- 0.13 l/lk. Theobromine is only slightly bound to plasma and milk proteins and concentrations in milk and saliva matched plasma data closely. The mean concentration ratios were 0.82 +/- 0.17 for milk/plasma and 0.92 +/- 0.17 for saliva/plasma. If a mother ate a 4-ounce chocolate bar every 6 hours and the infant nursed when the theobromine concentration in milk was at its peak, the infant could ingest about 10 mg of theobromine per day.