Corticosteroids bind to hippocampal glucocorticoid (GR) and mineralocorticoid (MR) receptors, thereby affecting behaviour and neurochemical transmission. Rat hippocampus has high levels of both receptors and their messenger RNAs (mRNA), but there is little information on receptors in human brain. We used in situ hybridization to determine the distribution of GR and MR mRNA expression in human hippocampus. Frozen sections of human postmortem hippocampus (5 patients, 58-88 years old, without cerebral pathology) were postfixed in paraformaldehyde and hybridized with 35S-UTP-labelled cRNA probes (transcribed in vitro from human cDNA subclones) under stringent conditions. Control included hybridization with sense probes and heterologous cRNA competition studies. GR mRNA was highly expressed in dentate gyrus, CA3 and CA4, but levels were significantly lower in CA1 and CA2. MR mRNA was also very highly expressed in hippocampus, with significantly higher levels in dentate gyrus and CA2, CA3 and CA4 than CA1. Controls confirmed the specificity of hybridization and there was little hybridization of sense probes. High GR and MR mRNA expression is found in both rat and human hippocampus but the subregional distributions clearly differ between the species.