We sought to determine whether agmatine (decarboxylated arginine), a bacterial product recently discovered for the first time in mammalian brain, was contained in other organs. A method was developed for isolation of agmatine from tissue and detection by RP-HPLC following solid-liquid extraction and derivatization with o-phthalaldehyde and mercaptoethanol. Recovery was about 80% and the limit of fluorometric detection was about 10 pg on column. In male Sprague-Dawley rats agmatine was unevenly and widely distributed in many tissues confirming its presence in mammals. The highest concentration (approximately 71 ng/mg net weight) was found in stomach, with aorta and small intestine next, followed by smaller levels in spleen, adrenal, aorta, and skeletal muscle and brain. Serum concentrations were high. Agmatine in male Long Evans rats of 3, 12, and 24 months of age demonstrated similar but not identical tissue distribution without any effect of aging. Since agmatine binds to alpha 2-adrenergic and imidazoline receptors, is bioactive in a number of tissues, is contained in neurons and is found in serum and tissues, the findings are consistent with a potential role for agmatine as a neurotransmitter and/or hormone. It also raises the possibility that agmatine may, as in bacteria, serve as a polyamine precursor along metabolic pathways previously not detected in mammals.