Despite of the increased availability of genetically modified mouse strains, the experimental models in the rat have provided the most widely employed and versatile models for the study of renal pathophysiology and functional genetics. The identification of the human gene mutated in the congenital nephrotic syndrome of the Finnish type (NPHS1) has recently been reported, and its protein product has been termed nephrin. Here we report the molecular cloning and characterization of rat nephrin cDNA. Rat nephrin cDNA has an open reading frame of 3705 bp, shows 82% sequence identity with human nephrin cDNA, and shows characteristic rat-specific splicing variants. The translated nucleotide sequence has 89% sequence identity at the amino acid level. The signal sequence, glycosylation, and cysteine localization patterns are nearly identical to those of human nephrin. As in the human, the rat nephrin transcript is expressed in a tissue-restricted pattern. Antipeptide antibodies raised to the intracellular nephrin-specific domain identified immunoreactivity exclusively within the rat kidney glomerulus by indirect immunofluorescence. Initial results with semiquantitative reverse transcriptase-polymerase chain reaction analysis showed a remarkable down-regulation of nephrin-specific mRNA in the puromycin nephrosis of the rat.