Lipid nanoparticles have received considerable attention in the field of drug delivery, due their ability to incorporate lipophilic drugs and to allow controlled drug release. Solid lipid nanoparticles (SLN), nanostructured lipid carriers (NLC), and nanoemulsion (NE) are three different lipid nanostructured systems presenting intrinsically physical properties, which have been widely studied in recent years. Despite the extensive applicability of lipid nanoparticles, the toxicity of these systems has not been sufficiently investigated thus far. It is generally believed that lipids are biocompatible. However, it is known that materials structured in nanoscale might have their intrinsic physicochemical properties modified. Thus, the aim of this study was to evaluate the cytotoxicity of these three nanoparticle systems. To this end, in vitro and in vivo toxicity studies were carried out. Our results indicate that nanoparticles containing the solid lipid GMS (SLN and NLC) induced an important cytotoxicity in vitro, but showed minimal toxicity in vivo--evidenced by the body weight analysis. The NE did not induce in vitro toxicity and did not induce body weight alteration. On the contrary, the SLN and NLC possibly induce an inflammatory process in vivo. All nanoparticle systems induced lipid peroxidation in the animals' livers, but only SLN and NLC induced a decrease of antioxidant defences indicating that the main mechanism of toxicity is the induction of oxidative stress in liver. The higher toxicity induced by SLN and NLC indicates that the solid lipid GMS could be the responsible for this effect. Nevertheless, this study provides important insights for toxicological studies of different lipid nanoparticles systems.