Locomotor sensitization to repeated ethanol (EtOH) administration is proposed to play a role in early and recurring steps of addiction. The present study was designed to examine the effect of agmatine on EtOH-induced locomotor sensitization in mice. Mice received daily single intraperitoneal injection of EtOH (2.5 g/kg, 20 v/v) for 7 consecutive days. Following a 3-day EtOH-free phase, the mice were challenged with EtOH on day 11 with a single injection of EtOH. Agmatine (10 to 40 μg/mouse), endogenous agmatine enhancers (l-arginine [80 μg/mouse], arcaine [50 μg/mouse], aminoguanidine [25 μg/mouse]), and imidazoline receptor agonist/antagonists were injected (intracerebroventricular [i.c.v.]) either daily before the injection of EtOH during the 7-day development phase or on days 8, 9, and 10 (EtOH-free phase). The horizontal locomotor activity was determined on days 1, 3, 5, 7, and 11. Agmatine (20 to 40 μg/mouse) administration for 7 days (development phase) significantly attenuated the locomotor sensitization response of EtOH challenge on day 11. Further, the agmatine administered only during EtOH-free period (days 8, 9, and 10) also inhibited the enhanced locomotor activity on the 11th day to EtOH challenge as compared to control mice indicating blockade of expression of sensitization. Daily treatment (i.c.v.) with endogenous agmatine enhancers like l-arginine (80 μg/mouse) or arcaine (50 μg/mouse) and aminoguanidine (25 μg/mouse) restrained the development as well as expression of sensitization to EtOH. Imidazoline I1 receptor agonist, moxonidine, and I2 agonist, 2-BFI, not only decreased the development and expression of locomotor sensitization but also potentiated the effect of agmatine when employed in combination. Importantly, I1 receptor antagonist, efaroxan, and I2 antagonist, idazoxan, blocked the effect of agmatine, revealing the involvement of imidazoline receptors in agmatine-mediated inhibition of EtOH sensitization. Inhibition of EtOH sensitization by agmatine is mediated through imidazoline receptors and project agmatine and imidazoline agents in the pharmacotherapy of alcohol addiction.