The total membrane-bound ATP hydrolytic activity in human epidermis is due to the activities of at least three differently located enzymes, namely Mg++-activated ATPase, phosphomonoesterase and adenyl cyclase. Cytochemical studies on psoriatic epidermis with various inhibitory and stimulatory substances showed reduced activities of ATPase and phosphomonoesterase, and a lack of sensitivity of adenyl cyclase to specific stimulators such as isoproterenol and glucagon. Since no differences of basal adenyl cyclase activity were observed between normal and psoriatic human skin without stimulation, it seems likely that in psoriasis a latent defect of adenyl cyclase may exist, resulting in a deficient response of this enzyme to regulatory agents. In conclusion, the present study reveals that not a single enzyme but the entire membrane-bound nucleotide metabolism is altered in psoriatic keratinocytes, causing a disturbance of the membrane-bound energy utilization, similar to findings in proliferating tumour cells.