Severe hyperhomocysteinemia (50-200 microM) often presents itself with acute neuronal dysfunction including seizures and psychosis. Its moderate form (15-50 microM) is associated with cognitive impairment and dementia. We investigated the neuropharmacological effects of homocysteine and its oxidized forms, homocysteinesulfinic acid (HCSA) and homocysteic acid (HCA), on neuronal network function utilizing dissociated cortical neurons from embryonic Wistar rats on microelectrode arrays. All substances inhibited dose-dependently and reversibly spontaneous neuronal network activity within seconds: L-HCSA and L-HCA blocked spontaneous spike rate (SSR) significantly at very low concentrations, with an IC50 of 1.9 and 1.3 microM, respectively; whereas the dose-response curve of D,L-homocysteine revealed an IC50 of 401 microM. These effects were antagonized by 2-amino-5-phosphonovaleric acid (APV) pointing to the NMDA receptor as mediator of this fast and reversible inhibition of network activity. We conclude that a neuronal dysfunction observed in hyperhomocysteinemia is likely due to HCSA and HCA since effective concentrations of homocysteine are not reached in patients.