Continuing usage of plastics worldwide has induced global environmental concern of their environmental impacts on biota¹. Among the variety of polymers used, eight constitute up to 95% of all primary plastics, among which polypropylene (PP) and polyethylene (PE) comprise 45-50% of the global production². Hence, PE microplastics (PE-MPs) are among the most commonly found in the environment³. Fish ingest MPs depending on their feeding ecology⁴. Due to the significance of size in MPs induced toxicity to fish⁵, the present study’s purpose was to verify the particle-size dependence of fish response to MPs. Thus, we exposed two freshwater fish species, zebrafish (Danio rerio) and perch (Perca fluviatilis), for 21 days to PE-MPs sized 10-45 μm and 106-125 μm. Thereafter, biochemical parameters and the metabolic profile were examined in liver and gills. Ex-vivo characterization by ATR-FTIR spectroscopy exhibited increased 10-45 μm PE-MPs concentration in the liver of both species, while 106-125 μm PE-MPs mostly concentrated in gills. The induced oxidative stress triggered changes in lipid peroxidation, DNA damage and ubiquitination and stimulated signal transduction pathways leading to autophagy and apoptosis. Smaller PE-MPs were more potent than the larger ones. In perch the metabolic changes in gills followed a size dependent pattern, indicating that stress conditions generate through different mechanisms. Taking into consideration the fact that MPs ingestion has been reported in more than 150 marine and freshwater fish species⁶, that MPs are incorporated through food transfer⁷, and thus MPs induced toxicity to fish is observed across multiple trophic levels⁵, it becomes apparent that MPs constitute a new pollutant that may enter the human food chain posing a threat to public health.