Exposure of mammalian skin to ultraviolet radiation (UVR) results in a transient inhibition of scheduled DNA synthesis. The wavelength dependence for this response was investigated in the epidermis of albino hairless mice. Groups of animals were exposed to narrow wavebands of UVR (HPBW 6.6 nm) from a monochromator in the 260-335 nm range. A dose-dependent inhibition of DNA synthesis was observed following exposure to all test wavebands except that centered at 335 nm. An action spectrum constructed from dose-response regression lines showed peak effectiveness at 290 nm. This spectrum bears a close resemblance to published action spectra for the induction of pyrimidine dimers in vivo, suggesting that DNA is the primary chromophore for both events. The DNA synthesis inhibition spectrum bears little resemblance to a published therapeutic action spectrum for the clearing of psoriasis.