A simple modification to an existing adult ventilator has been designed to permit mechanical ventilation of neonates and small children with the Ayre's T-piece circuit. The Nuffield Anaesthesia Ventilator Series 200 has been modified by replacing the piston in the patient valve with a fixed leak. This arrangement avoids the problems of critical adjustment commonly encountered with other methods of converting adult ventilators to paediatric use, and allows the ventilator controls to be used normally. The ventilator has been assessed according to the proposed International Standards Organization specifications for the evaluation of the performance of lung ventilators, and the results are described in detail. The modified ventilator is shown to perform as a time-cycled pressure generator capable of delivering tidal volumes between 10 and 300 ml at frequencies from 10 to 85/minute. It is therefore ideally suited to neonatal and paediatric use.