The effect of corticosterone on 5-HT1A receptor mRNA expression in different hippocampal cell fields was studied by in situ hybridisation. One week after adrenalectomy or chronic corticosterone treatment, the 5-HT1A receptor mRNA showed a negative, but not linear, correlation with plasma corticosterone levels over a wide concentration range, specifically in the dentate gyrus. The data suggest an involvement of the mineralocorticoid receptor in the regulation of the 5-HT1A receptor mRNA and a possible synergistic effect of additional glucocorticoid receptor activation.