This study showed that mitochondria isolated from animals studied in different seasons respond to G3 (100 渭M) exposure to a different extent. Probably, seasonal variations had the impact on rat metabolism and consequently on the received data. The used biological samples formed a heterogenous group and therefore the obtained results were not pooled together but treated separately. Nevertheless, the in vitro part of this study revealed that PAMAM G3 could be successfully used in the protection of heart mitochondria against MG-induced impaired respiratory activity. Despite these promising data, the protective effect of G3 was not confirmed in the in vivo experiment. This study revealed that dendrimer G3 (20 mg/kg bw) is toxic and very high mortality among the animals administered with G3 did not allow to perform a reliable data analysis.