Using in situ hybridisation, the effects of acute and chronic diazepam administration and diazepam withdrawal on preprocholecystokinin (CCK) mRNA levels in discrete regions of rat brain were determined. In cerebral cortex and a subpopulation of hippocampal neurones, CCK mRNA levels were increased after a single injection of diazepam and 24 h after withdrawal from chronic diazepam treatment, but not after chronic diazepam treatment. These results show that, in some neuronal groups, CCK mRNA expression is regulated by benzodiazepines, although there is no clear link between CCK mRNA levels and anxiety state.