4,5′,8-trimethylpsoralen photosensitizes the inhibition of the induction of nitrate reductase in XD cells of tobacco, by near ultraviolet light. The photosensitization depends on the concentration of 4,5′,8-trimethylpsoralen and the near ultraviolet light dose. Concomitant with the inhibition of nitrate reductase induction there is a severe inhibition of DNA and RNA synthesis. Oxygen uptake and protein synthesis are not affected. Translation of preexisting mRNA coding for nitrate reductase is insensitive to 4,5′,8-trimethylpsoralen and near ultraviolet light. It is concluded that the DNA of these cells is the target for the photochemical reaction of 4,5′,8-trimethylpsoralen. The template activity of the DNA is inhibited, and the expression of the genome thereby prevented. Translation of preexisting mRNA is not affected.