It has been shown that somatostatin inhibits the antidiuretic action of vasopressin in toad urinary bladder in vitro and in dogs and rats in vivo. The presence of somatostatin-like immunoreactivity (SLI) in the urinary bladder and kidney of the toad has suggested the possibility that somatostatin may act as a local paracrine hormone modulating the effect of vasopressin in the toad; however, the inability to localize SLI in mammalian kidney has raised doubt about the physiological role of somatostatin in the mammalian renal function. We identified SLI in rat kidney. Chromatography of rat kidney extracts of Sephadex G50 superfine revealed a single peak of SLI that co-eluted with somatostatin-14. Using the avidin-biotin-peroxidase conjugate technique, we localized SLI exclusively to small cells in the glomerulus with an estimated number of four to eight cells per glomerulus. Functional significance of somatostatin in the mammalian kidney is to be determined.