The GABA(A) receptor is a ligand-operated chloride channel. It has a pentameric structure. In mammalian brain different subunits are recruited from four gene subfamilies. Using immunocytochemistry, we investigated the distribution of the 13 GABA(A) receptor subunits in the hippocampus of the rat. GABA(A) receptor subunits were heterogeneously distributed within different hippocampal subfields. High concentrations of alpha1-, alpha2-, alpha4-, beta3-, gamma2- and delta-immunoreactivities were observed within the molecular layer of the dentate gyrus, representing the dendritic area of the granule cells. In the hippocampus proper, the predominant GABA(A) receptor subunits were alpha1, alpha2, alpha5, beta3 and gamma2 that were located throughout the strata radiatum and oriens of CA1 to CA3. Immunocytochemical staining was there less prominent for alpha4-, beta1-, beta2- gamma3- and delta- subunits. In the hippocampus proper, the beta1 subunit was preferentially located in CA2. The alpha4- and delta-subunits were somewhat more abundant in CA1 than in CA3. Numerous local circuit neurons in the hippocampus proper and the hilus of the dentate gyrus contained alpha1-, beta2-, gamma2- and/or delta-subunits. Alpha3 and gamma1 were present only in minute amounts and no alpha6-IR was detected in the hippocampal formation. The distribution of the GABA(A) receptor subunits indicates the existence of heterogenously constituted GABA(A) receptor complexes within various hippocampal subfields, which may exert different physiological or pharmacological properties upon stimulation by GABA or its agonists.