Using homology-based polymerase chain reaction (PCR) amplification, we demonstrate the presence of arginine decarboxylase mRNA in tissues involved in arginine metabolism (brain, kidney, gut, adrenal gland, and liver of the rat) but not in organs (lung, heart, and spleen) in which arginine metabolism is low or absent. The polymerase chain reaction product from the kidney had a nucleotide sequence 61% identical to that of the E. coli biosynthetic arginine decarboxylase. On a whole tissue basis, kidney homogenates were three times more active than brain homogenates at decarboxylating [1-14C]arginine. Subcellular fractionation localized the arginine decarboxylase activity of the kidney to the mitochondria fraction. Agmatine, one of the products of arginine decarboxylation, was found to inhibit nitric oxide formation by post-mitochondrial supernatants of the brain or kidney. We propose that arginine is metabolized to two structurally different signaling molecules, nitric oxide and agmatine. Furthermore, agmatine can influence the nitric oxide synthase pathway.