en Experimental research article Experimental research article <p>  The involvement of nitric oxide (NO) in the reinforcing properties of opioids was studied by examining the effect of a NO precursor (L-arginine) and NO synthase inhibitor (L-nitro-amino-methyl-ester, L-NAME) on the conditioned place preference (CPP) induced by morphine. The experiment was performed on male albino Swiss- White mice (25 ± 5 g). L-arginine in doses of 100, 200 and 500 mg/kg (i.p.) decreased CPP significantly, while L-arginine induced CPP in doses of 200 and 500 mg/kg <i>per se</i>. L-NAME doses of 0.75, 1.25, 2.5, 5, 10, 20 and 300 mg/kg (i.p.)had no effect on CPP induced by morphine, but injection of L-NAME (5 mg/kg) significantly decreased the effect of L-arginine on morphine-induced CPP. Therefore it may be concluded that NO may be involved in the psychological dependence to morphine. </p><p>  </p> Morphine; CPP; L-arginine; L-NAME 137 143 http://ppj.phypha.ir/browse.php?a_code=A-10-74-4&slc_lang=en&sid=1 Hedayat Sahrai 320031947532846001824 320031947532846001824 Yes Mohammad Reza Shariati 320031947532846001825 320031947532846001825 No Hasan Aghai-Firoozabadi 320031947532846001826 320031947532846001826 No Ali Khoshbaten 320031947532846001827 320031947532846001827 No Alireza Asgari 320031947532846001828 320031947532846001828 No Hasan Ghoshooni, 320031947532846001829 320031947532846001829 No