The transmission and toxicity of metals and metalloids were initially investigated with various nanofiltration (NF) and seawater reverse osmosis (SWRO) membranes, under oxic condition, using wastewater effluent and seawater samples. Various metals and metalloids were categorized into three groups according to their properties, such as specific gravity and concentration level, as well as others. The levels of various transition metals and metalloids included in the wastewater effluent and seawater samples were within ng/L-mu g/L range; the concentrations of almost all compounds were substantially reduced to ng/L or a few mu g/L levels after membrane filtrations, as measured by ICP-MS. Toxicity measurements were performed using bioassays, including both the Microtox and daphnia methods, for raw and membrane treated samples of wastewater effluent and seawater. The results indicated that (1) the SWRO membrane exhibited very good removal efficiencies for all metals and metalloids tested, (2) most metals and metalloids tested were efficiently removed by the tight NF membranes employed; however, some metals and metalloids included in seawater were not efficiently removed by the relatively loose NF membrane, (3) based on the Microtox measurements, only the seawater sample treated using the relatively loose NF membrane provided a relatively high toxic unit close to 1.0; otherwise, all other samples (both raw and membrane treated samples) exhibited relatively low toxicities.