The pharmacokinetics of caffeine were studied in young (age: 1 day, 7 days, 14 days, 30-45 days) and adult dogs following a single intravenous dose of 50 mg/kg. Mean (+/-SE) plasma elimination half life (T1/2) was 47.5 (+/-5.35) h in 1-day-old puppies, as opposed to 6.66(+/-0.85) h in adult dogs. A rapid decrease in plasma T1/2 values occurred during the first 2 weeks of life. At about 14 days of age caffeine plasma T1/2 was similar to that of adults. The volume of distribution was greatest (0.94+/- 0.03 liters/kg) and the total body clearance was smaller (2.81 +/- 0.73 liters/kg/min X 10(-4)) in the 1-day-old animals. The much smaller body clearance of caffeine in the newborn as compared to the adult dog could be due to a lower rate of metabolism and/or renal excretion of this drug in the young.