The aim of this study was to evaluate the toxic effect of nanosized ZnO, CuO and TiO(2) to Saccharomyces cerevisiae - a widely used unicellular eukaryotic model organisms in molecular and cell biology. The effect of metal oxide nanoparticles, their bulk forms and respective ionic forms were compared. The bioavailable Zn(2+) and Cu(2+) ions in the growth medium were quantified by recombinant microbial sensors. Nano and bulk TiO(2) were not toxic even at 20000 mg/l. Both, nano and bulk ZnO were of comparable toxicity (8-h EC(50) 121-134 mg ZnO/l and 24-h EC(50) 131-158 mg/l). The toxicity was explained by soluble Zn-ions as proved by the microbial sensor. However, nano CuO was about 60-fold more toxic than bulk CuO: 8-h EC(50) were 20.7 and 1297 mg CuO/l and 24-h EC(50) were 13.4 and 873 mg/l, respectively. The increase in toxicity of both CuO formulations at 24th hour of growth was due to the increased dissolution of copper ions from CuO over time. Comparison of EC(50) values of nano CuO, bulk CuO and Cu(2+) with bioavailable copper concentrations in the growth medium showed that the solubilized Cu-ions explained only about 50% of the toxicity of both, nano and bulk CuO. To our knowledge, this is the first study that evaluates the toxicity of ZnO, CuO and TiO(2) nanoparticles to S.cerevisiae.