We have studied the expression of fosB mRNA in the suprachiasmatic nucleus (SCN) of hamsters by in situ hybridization using oligonucleotides with sequences complementary to the C-terminal of the fosB mRNA sequence. In animals exposed for 48 h to darkness, there was little or no background expression in SCN cells of fosB mRNA at any circadian phase. Light pulses (30 min) were able to induce fosB expression only during the subjective night. Transcripts of fosB increased rapidly to peak by the end of a 30-min light pulse. Light-induced increases gradually declined in darkness, but levels were still elevated for up to 150 min after the light pulse. Induction in response to a light pulse was largely restricted to the ventrolateral portion of the nucleus which receives the heaviest retinal projection. The temporal and anatomical pattern of fosB mRNA expression in the hamster SCN therefore resembles that reported previously for other immediate-early genes, such as c-fos.