Effects of intra-central amygdala administration of L-arginine, a nitric oxide precursor, and NG-nitro-L-arginine methyl-ester (L NAME), a nitric oxide synthase inhibitor, on the morphine-induced sensitization and also on the expression of morphine-induced place conditioning in rats were studied. Subcutaneous (s.c.) administration of morphine (2.5, 5 and 7.5 mg/kg) induced place conditioning. Repeated pretreatment of morphine (5 mg/kg, i.p.) followed by 5 days no drug treatment, increased place conditioning induced by morphine (0.5 mg/kg). Repeated intra-central amygdala administration of L-arginine (0.3, 1 and 3 μg/ rat), with morphine during acquisition of sensitization, significantly increased or reduced morphine place conditioning in sensitized rats. The drug administration before testing also increased and reduced the expression of morphine place conditioning in sensitized animals. Repeated intra-central amygdala injections of L-NAME (0.3, 1 and 3 μg/rat) with morphine during acquisition of sensitization, reduced the acquisition of morphine place conditioning in the sensitized animals. The drug injection before testing also reduced morphine-induced conditioning. The results indicate that nitric oxide (NO) within the central amygdala may be involved in the acquisition and expression of morphine place conditioning in morphine-sensitized rats.