Attached bean leaves responded to thermal stresses, induced by microwaves in darkness, by lowering equilibrium leaf temperatures between 2 to 1O°C in still air. The lowering of equilibrium leaf temperatures of exposed water hyacinth plants in darkness were found to be even higher than 10°C in still air. The same behaviour was also observed for leaves which were placed in an air stream. The phenomenon is explained as transpirational cooling when stomates are opened in darkness in response to absorbed microwave radiation and high leaf temperatures.