Pharmacologically relevant factors such as enteral absorption, distribution, metabolism and excretion are age-dependent. The absorption of theophylline given in aqueous solution to prenatal infants with apnoea is markedly reduced when administered along with the infant's feed. The metabolic pathway of theophylline depends on the age group. Newborn and older infants form the pharmacodynamically active metabolite, caffeine. The main metabolites 1.3-dimethyl-uric acid and 3-methylxanthine are detectable, as in adults, but 1-methyl-uric acid remains below the demonstrable serum concentration level in infants of our study. The elimination velocity of theophylline is also dependent on the age. In order to achieve effective theophylline concentrations in the serum with oral preparations the galenic properties of the sustained-release products are decisive. By contrast, there was no difference between intravenous administration as permanent infusion or bolus injection. The data presented underline that in the treatment of apnea in premature infants as well as in the treatment of asthma in older children individual controls of serum concentrations are required to achieve further improvement of therapeutic success.