Soluble guanylate cyclase activity was measured in normal and psoriatic human epidermis. The specific activity of guanylate cyclase was determined to be increased 10-fold and 3-fold in involved and uninvolved epidermis of psoriatics, respectively, compared with normal epidermis. Arachidonic acid or HETE (5 to 50 microM) stimulated guanylate cyclase activity from involved epidermis 2 to 3-fold and from uninvolved epidermis up to 2-fold, but these fatty acids had no effect on the activity of this cyclase from normal epidermis. These results indicate that the biosynthetic capacity to generate cGMP is markedly increased in involved epidermis from psoriatics because of a markedly increased specific activity of guanylate cyclase and an alteration in a property of this enzyme activity which renders it stimulable by fatty acids reported to accumulate in this lesion.