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 Nomega-nitro-L-arginine (L-NA) into the hippocampus directly after the first trial. One hour later the discrimination performance of the animals was assessed. It was found that 10 microg and 30 microg, but not 3 microg, 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 microg and 30 microg) were observed. Since treatment with 30 microg 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.