beta-Defensins are epithelium-derived antimicrobial peptides that function in the host's innate defense. We identified the first member of the rat beta-defensin family, beta-defensin-1 (BD-1), in the kidney and determined its nucleotide sequence. It was predicted to be a 37-amino-acid peptide. Rat BD-1 mRNA was expressed most abundantly in the kidney, next in skin, tongue, esophagus, and uterus, followed (at low levels) by brain, trachea, stomach, urinary bladder, and ovary. BD-1 gene expression in rat kidney was not increased by lipopolysaccharide administration. BD-1 gene expressions in the kidneys of diabetic rodent models, cholecystokinin-insensitive Otsuka Long-Evans Tokushima Fatty rats, leptin-insensitive obese (fa/fa) Wistar rats, and db/db mice, were significantly lower than those of their lean littermates. BD-1 reduction may be in part responsible for the high incidence of urinary tract infections in diabetes mellitus.