Zinc oxide nanoparticles (ZnO-NPs) of different sizes and morphology were synthesized. The variables analyzed were zinc precursor concentration, the nature of the synthesis solvent and the concentration of surfactant agent. The solids synthesized were characterized using IR spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Considering the nature of the synthesis process, a tentative model of the mechanism of formation of the ZnO-NPs was proposed. ZnO-NPs with spheroidal morphology were obtained with a 10 mM CTAB concentration and these were selected to study their toxicity. For this study, Mus musculus mice were therefore given an oral dose of 50 mg ZnO/kg body weight (b.v.). Biopsies obtained from the livers and kidneys of the mice studied were analyzed using TEM and atomic absorption. The biopsy of the liver of the mouse given a dose of ZnO-NPs showed evidence of steatosis. The atomic absorption results showed the accumulation of Zn in both the liver and the kidney of the mouse.