The expiratory time (Te) and the rate of lung emptying in expiration are important determinants of functional residual capacity (FRC) in infants. In order to determine whether these factors also influence FRC in children during anesthesia, 20 preschool children were studied, 10 while receiving ketamine, and 10 during halothane anesthesia. Te was measured during quiet breathing and the passive expiratory time constant (tau) was determined from the passive expiratory flow volume (V/V) curve following a brief end inspiratory airway occlusion. The number of time constants available for expiration, Te/tau, was then calculated. The difference between FRC and the relaxation volume of the respiratory system (Vrs) (FRC-Vrs) was measured by extrapolating the linear segment of the V/V curve to zero flow, and measuring FRC-Vrs. During ketamine anesthesia, tau was markedly prolonged (1.15 s, range 0.73-2.29 s), with the result that Te/tau was, in all subjects, less than 2. Children anesthetized with halothane had shorter tau (0.38 s, range 0.24-0.65 s), and Te/tau was more than 2 in most subjects. FRC-Vrs was significantly greater in the subjects from the ketamine group (203 ml, range 115-392 ml) than in those from the halothane group (32 ml, range 1-71 ml). For the 20 subjects, there was a significant relationship between FRC-Vrs (ml) and Te/tau described by the equation. FRC-Vrs = 845.0e-1.28(Te/tau) The authors conclude that, in children during ketamine anesthesia, tau is prolonged and, in these children, the relationship of Te to tau is an important determinant of FRC-Vrs. Te/tau was not related causally to FRC-Vrs in the halothane group.(ABSTRACT TRUNCATED AT 250 WORDS)