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 per se. 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.