The level of nicotinamide adenosine dinucleotide (NAD) has been determined in the epidermis of 30 mice. Its value is 0.63 +/- 0.15 microgram/mg protein. Upon treatment with dimethylsulfate (DMS), the level of NAD drops in a dose-dependent fashion. This diminution is reversible when low doses of DMS are used. Upon irradiation with ultraviolet light, the level of NAD drops in the irradiated epidermis, the threshold of saturation being below 1200 J/m2. There is also a drop in the level of NAD in the epidermis protected against irradiation with a black rubber sheet.