Ornithine decarboxylase (ODC) was measured in the epidermis of healthy volunteers and the uninvolved skin of psoriatic patients at various times after sellotape stripping. Basal levels were less than 1 pmol/min/mg protein. Activity peaked to a maximum of 86 pmol/min/mg protein after 8 h; this was followed by an abrupt decline to a lower level which remained relatively constant for at least 36 h. No difference was seen between the response of controls and psoriatic patients. Pretreatment with topical corticosteroids reduced peak ODC levels to about one-half, but oral indomethacin had no effect.