The induced isomorphic response was used as a model in the study of the pathogenesis of psoriasis. Scraping to minute bleeding and freezing with Frigiderm induced a positive isomorphic response in 22 out of 49 patients, and in 48 out of 94 patients, respectively. These differences are not statistically significant. Pretreatment with lidocaine (Xylocaine), however, reduced significantly the number of positive responses. This effect is ascribed to the vasoconstrictor action of this substance. Dermal knife injury did not result in psoriatic lesions in the epidermis overlying the dermal cut. The intradermal placement of catgut produced a minimal inflammatory response in the dermis with no demonstrable change in the overlying epidermis. Hyaluronidase injected intradermally provoked moderate inflammatory reaction in the upper dermis with no elicitation of the isomorphic response. Chymotrypsin injected intradermally produced a severe inflammatory reaction in the mid- and upper dermis with no elicitation of the Koebner response. The dermal injury alone as employed in this study, therefore, was not sufficient to evoke an isomorphic response. There seems to be a relationship between the amount of trauma and appearance of the isomorphic response. This reaction is more likely to occur in patients with active psoriasis.