Expression of the gene coding for tryptophan oxygenase (TO) is switched on in rat liver about two weeks after birth. We identified two clusters of DNaseI hypersensitive (HS) sites in the TO gene upstream region; one near the promoter, the other at a distant upstream location (-8.5 kb). Hypersensitivity of upstream sites was present in adult and in 7 day old rat liver, but absent in kidney. To investigate their role in transcriptional regulation, a reporter gene controlled by both HS site regions was used to generate transgenic mice. In these animals the transgene followed the cell specific and developmental regulation of the endogenous gene: inactive after birth and active in adult liver. Transgenes containing only the promoter proximal HS site were non-functional.