The rate of increase of alveolar concentrations (FA/FI) of isoflurane and halothane was studied in children and adults during general anaesthesia and controlled ventilation. After 30 min of body equilibrium, elimination curves of the volatile anaesthetics were determined by measurement of alveolar (FA/FA0; infrared technique) and venous concentrations (gas chromatography). The distribution and elimination half-times (t1/2 alpha, t1/2 beta), clearance (Cl), volume of central and peripheral compartment (V1, Vz) and the volume of distribution at steady state (Vss) were calculated from the intercepts and slopes of a two-compartment model. During the uptake of anaesthetic concentrations of isoflurane and halothane, the FA/FI ratio of each gas was found to rise significantly faster in children than in adults. The reason for the more rapid approach to equilibrium in children seems to be related to physiological differences. Irrespective of age, uptake of isoflurane was more rapid than that of halothane, as it is less soluble. Similarly, isoflurane was eliminated from the lung or blood faster than halothane. Moreover, anaesthetic wash-out in children differed from that in adults. In the paediatric age group t1/2 beta under isoflurane was shorter than in adults, whereas halothane excretion took longer in children. This could be accounted for by the larger volumes of distribution Vz and Vss in the young, due to higher organ affinity of halothane. From our data we conclude that age significantly affects uptake and elimination of volatile anaesthetics and the control of anaesthesia is easiest with isoflurane in paediatric patients.