An advanced controlled-ventilation, open-top chamber (CVOTC) has been designed which offers considerable advantages over the current continuously-ventilated open-top chambers (OTC), used in climate change impact studies on plants. The CVOTC gives a five-fold reduction in carbon dioxide consumption by recirculating the air when ventilation is not required to control temperature. Full ventilation at six air changes per minute limited the chamber temperature to 2°C above ambient during periods of high solar flux. The average temperature excess was 1·6°C. The air distribution system within the CVOTC provides uniform air speeds above 1 m s-1which produces leaf boundary layer resistances comparable with those of plants in the open. A mean chamber light transmission of 90% in direct sunlight was attained using inexpensive film cladding and by employing a simple chamber frame. These performance improvements derive from the efficient design of the chamber and control of ventilation/recirculation using a novel, scheduled-gain, adaptive control system.