Nicotine produces dose-dependent enhancement of performance in an active avoidance test, and also increases the levels of NO2- and NO3-, which are stable metabolites of nitric oxide (NO), in various brain regions of rats. On the basis of these two observations, we hypothesized that the beneficial effect of nicotine on learning could result from increased NO in relevant brain regions. We therefore tested active avoidance performance in rats given L-N-omega-nitroarginine (L-NA) to inhibit NO synthetase (NOS) prior to nicotine administration, Male Sprague-Dawley rats received L-NA (30 or 50 mg/kg), nicotine (0.4 mg/kg), saline or combinations of these treatments before learning trials. Rats were also tested on the inclined plane, to assess the possible effects due to impairment of motor function by drug treatments on active avoidance learning. L-NA treatment impaired the acquisition of active avoidance learning, and this defect was partially overcome by the co-administration of nicotine. Nicotine facilitated learning and significantly increased the number of correct responses. The threshold for the effect of NOS inhibition on performance exceeded 30 mg/kg L-NA, whereas 50 mg/kg impaired learning and also eliminated the nicotine-induced enhancement of learning. On the fifth day of learning trials, no facilitation of learning by nicotine was observed in rats receiving either dose of L-NA. Our results suggest that NO is involved in the facilitation of active avoidance learning by nicotine. (C) 2000 Lippincott Williams & Wilkins.