Epidermal nicotinamide adenine dinucleotides were measured in subcorneal and basal epidermal layers in patients with psoriasis and in healthy controls during a 2-week period in which they were treated once a day with 0.15% dithranol in white petrolatum. Lowry's microtechniques utilizing enzymatic cycling were used. In the controls the total NAD contents decreased 25% during the treatment period. The total NADP content did not change, nor did the proportions of NADH and NADPH. In the psoriatic patients effects of the treatment were seen only in the epidermis of the lesions. Both the total NAD and NADP displayed an initial increase on the second day, followed by a steady decrease to the levels found in the non-involved skin. During the first week of treatment parakeratosis was still evident in the lesions. In spite of variation in the total NAD contents during this period, a significant reduction of the percentage of NADH was found. Concomitant with the appearance of an orthokeratotic horny layer, both NAD+ and NADH returned to normal. The percentage contribution of NADPH did not change during these events.