The distribution of somatostatin-like immunoreactive (SS-LI) material and its colocalization with glutamic acid decarboxylase (GAD)-like immunoreactivity were studied in the rat hippocampus and dentate gyrus neurons using immunohistochemistry. In the dentate gyrus and CA1 region, SS-LI perikarya were concentrated in the hilus and in the stratum oriens, respectively, whereas immunoreactive cell bodies were rarely seen in other layers. Approximately half of the SS-LI neurons of the CA3 region were situated in the stratum oriens, the other half being scattered in strata pyramidale, lucidum and radiatum. About 90% of SS-LI neurons were also GAD-like immunoreactive, whereas about 14% of GAD-like immunoreactive (GAD-LI) neurons were SS-like immunoreactive. The percentage of GAD-LI neurons which were also immunoreactive for SS varied from one layer to the other. This percentage was about 30% in the hilus of the dentate gyrus and in the stratum oriens of the CA1 and CA3 regions; it was 5-10% in the strata pyramidale, lucidum and radiatum of the CA3 region and reached only 2% in the granule cell layer and molecular layer of the dentate gyrus and in the stratum pyramidale and stratum radiatum in the CA1 region. These observations indicate that the majority of SS-LI neurons in the rat hippocampal formation are a subpopulation of GABAergic neurons.