Arsenite and its metabolites, dimethylarsinic or dimethylarsinous acid, have previously been shown to disturb synaptic transmission in hippocampal slices of rats (Krüger, K., Gruner, J., Madeja, M., Hartmann, L.M., Hirner, A.V., Binding, N., Mubetahoff, U., 2006a. Blockade and enhancement of glutamate receptor responses in Xenopus oocytes by methylated arsenicals. Arch. Toxicol. 80, 492-501, Krüger, K., Straub, H., Binding, N., Mubetahoff, U., 2006b. Effects of arsenite on long-term potentiation in hippocampal slices from adult and young rats. Toxicol. Lett. 165, 167-173, Krüger, K., Repges, H., Hippler, J., Hartmann, L.M., Hirner, A.V., Straub, H., Binding, N., Mubetahoff, U., 2007. Effects of dimethylarsinic and dimethylarsinous acid on evoked synaptic potentials in hippocampal slices of young and adult rats. Toxicol. Appl. Pharmacol. 225, 40-46). The present experiments investigate, whether the important arsenic metabolites monomethylarsonic acid (MMA(V)) and monomethylarsonous acid (MMA(III)) also influence the synaptic functions of the hippocampus. In hippocampal slices of young (14-21 days-old) and adult (2-4 months-old) rats, evoked synaptic field potentials from the Schaffer collateral-CA1 synapse were measured under control conditions and during and after 30 and 60 min of application of the arsenic compounds. MMA(V) had no effect on the synapse functions neither in slices of adult nor in those from young rats. However, MMA(III) strongly influenced the synaptic transmission: it totally depressed the amplitudes of fEPSPs at concentrations of 50 micromol/l (adult rats) and 25 micromol/l (young rats) and LTP amplitudes at concentrations of 25 micromol/l (adult rats) and 10 micromol/l (young rats), respectively. In contrast, application of 1 micromol/l MMA(III) led to an enhancement of the LTP amplitude in young rats, which is interpretable by an enhancing effect on NMDA receptors and a lack of the blocking effect on AMPA receptors at this concentration (Krüger, K., Gruner, J., Madeja, M., Hartmann, L.M., Hirner, A.V., Binding, N., Mubetahoff, U., 2006a. Blockade and enhancement of glutamate receptor responses in Xenopus oocytes by methylated arsenicals. Arch. Toxicol. 80, 492-501). These effects are probably not mediated by changes in cell excitability or in presynaptic glutamate release rates, since antidromically induced population spikes and paired-pulse facilitation failed to show any MMA(III) effect. The impairment of the excitatory CA1 synapse is more likely caused by the action of MMA(III) on postsynaptic glutamatergic receptors and may be jointly responsible for dysfunctions of cognitive effects in arsenic toxicity.