London SW3 6LX 1 L-NG-nitro arginine methyl ester (L-NAME, 1-75mgkg-1) administered intraperitoneally (i.p.) elicits dose-related antinociception in the mouse assessed by the formalin-induced paw licking procedure. Antinociceptive activity is still present 24 h after injection. L-NAME (75 mg kg 1, i.p.) is also antinociceptive in the acetic acid-induced abdominal constriction and hot plate procedures. 2 L-NAME additionally produces a dose-related inhibition of formalin-induced paw licking following intracerebroventricular (i.c.v., 0.1-I00jig per mouse) and oral (p.o., 75-lSOmgkg -) administration.3 L-Arginine (600mg kg-, i.p.) but not D-arginine (600mg kg 1) or naloxone (5 mgkg ')reverses the antinociceptive effect of L-NAME in the formalin test. 4 High doses of L-NAME (37.5-600mgkg-1) but not D-NAME (75mgkg-1) administered i.p. produce dose-related increases in blood pressure of the urethane-anaesthetized mouse whilst i.c.v. injected L-NAME (0.1 and lOO1pg per mouse) is inactive.5 L-NAME (75 mgkg-1, i.p.) did not inhibit oedema formation in the formalin-injected mouse hindpaw. 6 L-NAME (75mgkg-1) did not produce any overt behavioural changes in treated mice and failed to influence locomotor activity or the incidence of dipping, crossing, rearing or circling behaviour assessed by a modified 'head-dipping' board procedure. A high dose of L-NAME (600mgkg-1) reduced dipping behaviour and locomotor activity suggesting a possible sedative effect. D-NAME (600mgkg 1) was inactive. 7 These results suggest that L-NAME produces an opioid-independent and long-lasting antinociception in the mouse most probably by a direct effect within the central nervous system.

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