The role of lithium in treating bipolar affective disorder is poorly understood; however, it may involve effects on brain 5-HT function. We have shown that the 5-HT2A/2C receptor agonist DOI (2,5-dimethoxy-4-iodophenylisopropylamine) induces the expression of c-fos in rat brain which correlates with the distribution of 5-HT2A receptors. We now report on the effect of lithium on 5-HT receptor activation. Rats were treated chronically with dietary lithium before being given either DOI or the 5-HT1A receptor agonist 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino) tetralin), and their brains were processed for c-fos immunohistochemistry. Lithium treatment greatly enhanced levels of Fos seen after DOI, but not after 8-OH-DPAT; layer II of caudal piriform cortex, previously devoid of staining, exhibited the most marked labelling. This suggests that chronic lithium selectively alters immediate-early gene expression in brain. Such alteration may underlie the action of lithium in treating bipolar affective disorder.