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The release in aquatic environments of emergent contaminants such as copper oxide nanoparticles (CuO-NPs) has generated concerns on their short- and long-term toxicity and the potential risk for more vulnerable animal groups, such as amphibians. In this sense, the aim of this work was to evaluate the toxicity of CuO-NPs in comparison with its respective salt (CuCl 2 ) in embryos and larvae of a native amphibian, Rhinella arenarum, by acute (96 h) and chronic (504 h) standardized bioassays. Lethality and sublethal effects such as developmental, morphological, and ethological alterations were assessed in a wide range of concentrations (0.001-100 mg/L). Neurotoxic effects by acetyl (AChE) and butyrylcholinesterase (BChE) activity levels and changes in the lipid content were also assessed at sublethal concentrations. Results showed that CuCl 2 caused higher lethality than CuO-NPs in both developmental periods. Embryos were more sensitive than larvae with LC50-96 h = 0.080 mg CuCl 2 /L and 1.26 mg CuO-NPs/L and 0.21 mg CuCl 2 /L and 20.17 mg CuO-NPs/L, respectively. At acute exposure, embryos exhibited several developmental abnormalities such as developmental delay, edema, axial flexure, and microcephaly. Larvae presented spasmodic contractions and weak movements. Regarding neurotoxicity, a significant increase in AChE activity at low concentrations as well as an inhibition of BChE activity at all tested concentrations was evidenced for both substances at acute exposure. Moreover, an increment in phospholipid and triglyceride levels was observed at the highest concentration of CuO-NPs (10 mg/L) at chronic exposure. The chromatographic separation of lipids showed no apparent differences in acylglycerols and free fatty acid bands, between the treatments and the control. The differences in toxicity between CuO-NPs and CuCl 2 could be due to structural and physicochemical characteristics that influence their bioavailability and toxicity. Considering the exponential growth in the production and use of these substances, it is expected that the levels of contamination will rise considerably in the future, so that wildlife, particularly aquatic organisms, will be more increasingly exposed, representing a potential risk for their populations., Competing Interests: Declarations. Ethical approval: The experimental procedures were approved by the Animal Care and Use Committee of the National University of San Martin (CICUAE-UNSAM, Res. 11/2021). Consent to participate: Not applicable. Consent for publication: Not applicable. Competing interests: The authors declare no competing interests., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)