Many biological properties have been attributed to ruthenium complex I (trans-[RuCl(2)(nic)(4)]) and ruthenium complex II (trans-[RuCl(2 )(i-nic)( 4)]) including nitric oxide synthase inhibition. In this study, we evaluated pharmacological effects of these complexes on anxiety and memory formation. Memory was evaluated with inhibitory avoidance and habituation to an open-field and anxiety was tested with elevated plus-maze. Adult male Wistar rats (250 to 350 g) received intraperitoneal injections of vehicle, ruthenium complex I (45.2, 90.4, or 180.7 mumol/kg), or ruthenium complex II (0.08, 4.5, or 13.6 mumol/kg) 30 min prior open-field training or elevated plus-maze test and 30 min or 0 h after training. No effects were observed in the anxiety parameters and habituation to an open-field. The ruthenium complexes impaired memory retention compared with vehicle group in the inhibitory avoidance, as when administrated 30 min prior as immediately after training. The memory impairment induced by ruthenium complexes may be due to their nitric oxide synthase inhibition capacity.