Standards for artificial pulmonary ventilation at ventilatory frequencies of 15 and 20 cycles per min, (PETCO2 4 +/- 0.5 kPa), were defined in 16 children with healthy lungs and body weights between 2.6 and 22.6 kg. A tidal volume ventilator and balanced anaesthesia were used during abdominal surgery. At f = 20 cycles/min an approximately direct proportionality existed between tidal volume and body weight (VTpat = 12.3 . kg b.w.--2.1, r = 0.99). The mean value VTpat per kg b.w. was 12.1 +/- 1.4 ml/kg. At f = 15 cycles/min, a linear relationship between tidal volume and kg b.w. was recorded (VTpat = 14.6 . kg b.w. + 16, r = 0.97). The mean value VTpat per kg b.w. was 15.9 +/- 1.5 ml/kg. A significant decrease in total compliance was recorded during the initial part of the surgical procedure. A rectilinear relationship existed between total compliance and kg b.w. (CTOT = 11.2 . kg b.w.--12.5, r = 0.95). The mean value CTOT per kg b.w. was 9.7 +/- 0.54 ml/kPa/kg at f = 20. The endotracheal peak pressure maintained a fairly constant pressure level independent of the initial pressure within the ventilator, the tidal volume and the body weight, a phenomenon explained by the hypothesis that the proportional increase in total lung volume and functional residual capacity in the older children compensated for the higher pressures.