The basic helix-loop-helix (bHLH-PAS) proteins aryl hydrocarbon receptor nuclear translocator (Arnt) and Arnt2 are transcriptional regulators that function as dimerizing partners for several bHLH-PAS proteins and also some nonrelated partners. They are involved in various biological functions, including regulation of developmental genes. In earlier studies, the developmental expression of Arnt was reported to be almost ubiquitous, whereas Arnt2 expression has been shown to be more limited, comprising neuronal tissues as the main site of expression. Here we provide a detailed description of the expression of Arnt and Arnt2 mRNA in mouse tissues during embryonic and early postnatal development. Arnt and also Arnt2 transcripts, in contrast to earlier reports, are shown to be expressed more widely during development yet show a temporally and spatially specific pattern.