Abstract: Radioimmunoassay of plasma caffeine in 15 healthy newborn infants indicated that the half-life of caffeine in these full-term infants, as in premature infants, is much longer than in older children or adults. Half-life ranged from 31-132 hours, with a mean of 82 hours. These slow clearance times for caffeine are indicative of a deficit in oxidative metabolism which is not corrected until the infant matures. Since caffeine is acquired transplacentally from the mother, its pharmacologic actions could persist in the infant for several days after birth even without additional caffeine intakes through breast milk. Thus, the long-half life of caffeine in the newborn infant may be clinically significant.