We have cloned a cDNA encoding rat neutrophil inducible nitric oxide synthase (iNOS). The amino acid sequence of the iNOS in the rat neutrophils was 95% identical to that in the mouse macrophages, and the difference between the two sequences was most likely the result of species variability. Moreover, the iNOS in the rat neutrophils was 99% identical in amino acid sequence to that in the rat hepatocytes and vascular smooth muscle cells (VSMC). Northern blot analysis of total RNA from both rat macrophages and neutrophils showed a single band above 4 kilo-nucleotides. These findings suggest that iNOS in rat neutrophils, macrophages, hepatocytes, and VSMC are almost identical in the primary structure.