Nitric oxide (NO) synthase, the enzyme responsible for NO formation, is located in brain regions such as amygdala and dorsolateral central grey, regions which are known to be involved in anxiety. To investigate the possible role of NO in anxiety, rats received acute i.p. injections of NG-nitro-l-arginine (L-NOARG, 7.5-120 mg kg-1), an inhibitor of NO synthase, and were tested in the elevated plus maze, an animal model of anxiety. The drug, at doses of 30-120 mg kg-1, decreased the percentage of entries and time spent on the open arms of the maze, but these doses, with exception of 30 mg, also decreased the number of entries into enclosed arms. These effects disappeared when the animals were tested after chronic L-NOARG treatment (3.75 to 60 mg kg-1 i.p., twice a day for four days). The effects of acute i.p. injection of 30 mg kg-1 of L-NOARG were blocked by i.c.v. pretreatment with 1000 nmol of l-arginine (but not 500 nmol). Thus, inhibition of NO formation in the central nervous system seems to decrease exploration of the elevated plus maze, an effect that disappears after four days of chronic (twice a day) L-NOARG administration.