The study was designed to test whether blockade of endogenous NO production affects learning and memory formation in rats. The rats received an intracerebroventricular (i.c.v.) injection of the inhibitor of nitric oxide synthase, N omega-nitro-L-arginine (NAME), 30 min before each training, and were then tested in the one-trial passive avoidance response and Morris water maze task, respectively. The results showed that: (1) among four concentrations of drug used, only doses higher than 3 mumol of i.c.v. NAME impaired learning and memory formation significantly (p < 0.05) in both test models; (2) in Morris water maze task, the animals treated with highest dose of NAME (5 mumol) failed to learn while those treated with lower doses succeeded; (3) in Morris water maze task, the i.c.v. NAME, even in higher doses, did not affect the capacity of finding a visible platform. It is suggested that NO is involved in learning and memory formation by potentiating or facilitating mainly the acquisition process.