<p>Electrophysiology is an emerging approach in the context of environmental toxicology, where it represents a non-invasive biomarker that has been little studied in tropical fish. In this study, we evaluated electrocardiographic (ECG) and electromyographic (EMG) responses in <i>Hyphessobrycon heterorhabdus</i>, a native Amazonian species considered a promising biological model for ecotoxicological studies, and Menthol was selected as the pharmacological model for methodological validation in this species. Fish were exposed to 30 mg L^− 1 for three minutes and subsequently transferred to clean water to assess recovery. The ECG showed a reduction in heart rate and an increase in the wave intervals in fish exposed to menthol. The EMG recorded the occurrence of transient muscle spasms before a progressive decrease in contraction strength as anesthesia deepened. After recovery, both ECG and EMG tracings returned to baseline conditions, indicating the reversibility of menthol-induced anesthesia. Thus, electrophysiological markers in <i>H. heterorhabdus</i> proved to be effective tools for detecting short-term physiological changes and supports their application in studies involving environmental stressors or chemical exposures.</p>