The use of methylxanthine molecules labeled with stable isotopes (15CN and 13C) gives the following results: 1. The biotransformation of theophylline to caffeine by N-7 methylation occurs in premature newborns treated with this methylxanthine. 2. It permits us to differentiate with perfect selectivity the two origins of caffeine: exogenous caffeine coming from breast feeding and endogenous caffeine coming from the infant's metabolism. 3. It allows us to measure the placental transfer of theophylline administered to the mother and to show that the fetus can metabolize this xanthine into caffeine.