Five commercial glasshouses, each with a floor area of 10 m × 5 m, are equipped for control of temperature, humidity, carbon dioxide concentration and ventilation rate using a PDP 11 computer. Heating is provided by a hot-air gas-fired system with a fast response time and roof ventilators are mechanically operated. Humidity is increased by the injection of fine water droplets. The internal concentration of CO2 is held close to the external concentration by the injection of pure CO2 during the day. Air temperature is allowed to cycle in phase with the external temperature and an algorithm applies retrospective corrections which hold long-term mean temperatures within about 0.1 ° C of set values. Control of saturation deficit is limited by the vapour pressure of the external air and by the temperature of the glasshouse walls. The dependence of crop photosynthesis rate on irradiance has been established from diurnal changes in the rate of CO2 injection. Each glasshouse is large enough to allow regular harvesting of plants and the installation of tube solarimeters measuring light interception, access tubes for neutron probes, tensiometers and auxanometers. The cost of the system was about £500 per m2, an order of magnitude less than figures quoted for conventional growth cabinets and phytotrons.