It has been suggested that agmatine (decarboxylated arginine) is an endogenous clonidine-displacing substance (CDS) which recognizes alpha 2-adrenoceptor and non-adrenoceptor, imidazoline binding sites. We have examined the effect of agmatine at alpha 2-adrenoceptor binding sites and pre- and postjunctional alpha 2-adrenoceptors. Agmatine produced a concentration-dependent inhibition of 1 nmol/l 3H-clonidine binding to both rat (pKi-5.10 +/- 0.05) and bovine (pKi-4.77 +/- 0.38) cerebral cortex membranes. However, agmatine (0.1-100 microM) failed to activate pre-junctional alpha 2-adrenoceptors regulating transmitter release in the guinea-pig isolated ileum and rat isolated vas deferens, nor did it activate postjunctional alpha 2-adrenoceptors of the porcine isolated palmar lateral vein which mediate contraction or inhibition of forskolin-stimulated cyclic AMP formation. High concentrations of agmatine (10-30-fold the pKi at alpha 2-adrenoceptor binding sites) failed to influence alpha 2-adrenoceptor activation by either clonidine or UK-14304 (5-bromo-6-[2-imidazolin-2-ylamino]-quinoxaline bitartrate) in any of the peripheral preparations examined. Moreover, even in a preparation where an interaction with alpha 2-adrenoceptor binding sites on cell membranes can be demonstrated, the rat cerebral cortex, agmatine failed to inhibit forskolin-stimulated cyclic AMP in the intact tissue or affect the inhibition produced by the selective alpha 2-adrenoceptor agonist UK-14304. Agmatine was also devoid of agonist activity in two preparations, the rat isolated thoracic aorta and the rat isolated gastric fundus, in which CDS has been reported to produce non-adrenoceptor effects. Thus, we have confirmed that agmatine recognizes alpha 2-adrenoceptor binding sites and, therefore, is a CDS.(ABSTRACT TRUNCATED AT 250 WORDS)