IN order to study the changes undergone by ergosterol under the influence of ultra-violet irradiation, the following arrangement has been devised: The flat platinum surface of a sensitive platinum-tellurium thermopile (constructed by Dr. W. Vanselow in this laboratory) was coated with a layer of solid ergosterol cemented with ether and was then exposed to monochromatic radiation from an intense quartz-mercury arc using a Hilger quartz monochromator. Variations in the intensity of the light source were checked by the use of a second thermopile.