Acon-trolled-environment unit consisting of eight chambers is described. An attached darkroom and enclosure of the chambers in a darkroom allow experiments in the physiology of flowering. Temperature range (from below 0[degree]C to 45[degree]C or more) and light intensities (2,000-5,000 ft-c) allow limited experiments in physiological ecology. Temperature is controlled by modulating the rate of flow of chilled brine or hot water through coils in each chamber. Exceptionally large coils and the brine system offer some special advantages. For example, freezing temperatures may be obtained in the chambers without frosting the coils (provided brine temperature is as high as possible), and the coils do not remove excessive moisture from the air. Accurate humidity control is yet to be achieved. The lighting system is a conventional one, consisting of mixed fluorescent and incandescent light. Separation of the lights from the chambers by a layer of plastic is essential for proper control of lamp ambient temperature. Plastic also results in nearly double the flowering response in Xanthium. Special light sources for high-intensity light and other special features are mentioned.