FK 506 and cyclosporin A (CyA) are two immunosuppressive drugs which are known to be effective in the treatment of psoriasis by inhibiting the activation of T cells. In contrast, their influence on the proliferation of keratinocytes is discussed controversially. The second messenger cyclic adenosine monophosphate (cAMP) has been regarded as a regulator for cell growth and proliferation for 20 years. Hyperproliferation of many cells and particularly of psoriatic keratinocytes was speculated to be due to a decrease in cAMP levels in the psoriatic epidermis, whereas new findings could not confirm these observations. To clarify this discussion we determined the intracellular cAMP content in isoprenaline-stimulated keratinocytes from psoriatics and controls after treatment with CyA or FK 506. Ethanol and the β-blocking drug propranolol served as controls. The basal level of cAMP and the response to isoprenaline in psoriatic keratinocytes did not differ from those of controls. CyA dramatically reduced the cAMP level and FK 506 just slightly diminished it in a dose-dependent manner. Both drugs diminished the cAMP level more effectively in the keratinocytes from lesional psoriatic skin than in keratinocytes from controls. These data provide evidence that CyA influences early signal transduction pathways by depressing the intracellular cAMP in keratinocytes. This supports the view of other groups that CyA and perhaps also FK 506 influence not only immunocompetent cells but also keratinocytes in the treatment of psoriasis. Furthermore, it is doubtful that a low cAMP level is a positive regulator for cell growth and the hyperproliferation of psoriatic keratinocytes.