Salicylate excretion was studied in the breast milk of a nursing mother (Patient A) taking chronic therapeutic doses of aspirin, and caffeine excretion was monitored in the breast milk of a nursing mother (Patient B) who was a heavy coffee drinker. Salicylate concentrations were maximal in serum at 2.25 hours (10.8 mg/dL) and in milk at 3.00 hours (1.0 mg/dL) following 975 mg of aspirin in Patient A. Caffeine concentrations peaked at 5.50 hours in serum (2.14 micrograms/mL) and at 2.00 hours in milk (1.15 micrograms/mL) during a period of steady coffee drinking by Patient B. Milk:serum concentration ratios ranged up to 0.08 for patient A and up to 0.63 for Patient B, demonstrating that relatively more caffeine than salicylate was excreted into milk. Sodium, potassium, pH, and percent solute remained essentially unchanged in milk samples from both patients throughout the study periods so that changes in the state of hydration of pH of the milk could not be implicated for the observed excretion patterns. More than 25 liters of milk at its peak drug concentration would have to be consumed by the infants of Patients A and B respectively to provide the salicylate content of one aspirin tablet or the caffeine content of an average cup of coffee.