OBJECTIVE The effect of agmatine on prostate contractility as well as the roles of imidazoline receptors and potassium channels in this action were studied using isolated Wistar rat prostate tissue. METHODS Rat prostate strips were pre-contracted with 1 µmol/L phenylephrine or 50 mmol/L KCl. The relaxation response to agmatine (1-100 µmol/L) was measured. The effects of imidazoline receptor blockers: efaroxan, BU224, KU14R; ATP-sensitive K(+) channels (KATP ) channel inhibitor: glibenclamide; cyclic AMP (cAMP) phosphodiesterase inhibitor: IBMX; or protein kinase A (PKA) inhibitor: H-89 on the agmatine-induced relaxation were studied. RESULTS Agmatine produced relaxation in prostate strips pre-contracted with phenylephrine or KCl in a dose-dependent manner. This relaxation was significantly reduced by BU224, a selective I2 imidazoline receptor (IR) blocker, but not by I1 or I3 IR blockers (efaroxan, KU14R respectively). Moreover, the agmatine-induced relaxation was attenuated by glibenclamide and H-89, but enhanced by IBMX. CONCLUSIONS The results suggest that agmatine causes rat prostate relaxation by activation of the I2 IR, which opens KATP channels through cAMP/PKA pathway.