Nanotechnology is an emerging field that involves the development, manufacture and measurement of materials and systems in the submicron to nanometer range. Its development is expected to have a large socio-economical impact in practically all fields of industrial activity. However, there is still a lack of information about the potential risks of manufactured nanoparticles for the environment and for human health. In this work, we studied the cytotoxicity, genotoxicity and morphological transforming activity of cobalt nanoparticles (Co-nano) and cobalt ions (Co(2+)) in Balb/3T3 cells. We also evaluated Co-nano dissolution in culture medium and cellular uptake of both Co-nano and Co(2+). Our results indicated dose-dependent cytotoxicity, assessed by colony-forming efficiency test, for both compounds. The toxicity was higher for Co-nano than for Co(2) after 2 and 24 h of exposure, while dose-effect relationships were overlapping after 72 h. Statistically significant results were observed for Co-nano with the micronucleus test and the comet assay, while for Co(2+) positive results were observed only with the latter. In addition, even when Co-nano was genotoxic (at >1 microM), no evident dose-dependent effect was observed. Concerning morphological transformation, we found a statistically significant increase in the formation of type III foci (morphologically transformed colonies) only for Co-nano. Furthermore, we observed a higher cellular uptake of Co-nano compared with Co(2+).