IN the present study we investigated the consequences of hippocampal nitric oxide synthase (NOS) inhibition on the performance in an object recognition task in rats. In a first study we injected Nω-nitro-L-arginine (LNA) into the hippocampus directly after the first trial. One hour later the discrimination performance of the animals was assessed. It was found that 1 μ g and 3 O°g, but not 3 μg, L-NA impaired the performance of the rats. In a second study in which we injected L-NA 45 min before the first trial no effects of treatment (10 μg and 3 Oμg) were observed. Since treatment with 30 μg has been found to inhibit hippocampal NOS almost completely and lasts longer than 2 h, it was concluded that hippocampal NOS inhibition induced a state-dependent performance deficit. Consequently, studies that examine the effects of NOS inhibition on cognitive functions should take this confounding effect into account.