The aim of this study was to evaluate the anaesthetic effect of MS-222 and propofol and determine their optimal concentrations for safe handling of the tetra Astyanax altiparanae in the laboratory. The fish were separated by length into three classes: I (1.5-5.0 cm), II (5.1-8.0 cm) and III (greater than 8.1 cm). Pilot tests were performed to evaluate the appropriate anaesthetic concentrations for inducing the five possible anaesthetic stages: I -sedation; II -light anaesthesia; IIIdeep anaesthesia; IV -surgical anaesthesia; and V -spinal collapse. After defining the maximum and minimum concentrations required to induce stage IV anaesthesia, the animals were exposed to five intermediate concentrations (n = 10 fish/concentration) of each anaesthetic for 15 min. The animals were then transferred to clean water to evaluate the time required for recovery. In addition, blood glucose levels were measured for class II and class III fish subjected to the previously defined ideal concentrations for each of the tested anaesthetics (n = 10 fish/treatment). Both evaluated substances are suitable to anaesthetize A. altiparanae. The optimal MS-222 concentration was 90 mg L À1 , and this result was similar for all three size classes. The optimal propofol concentrations for inducing surgical anaesthesia in the size classes I, II and III were 0.22, 0.23 and 0.27 respectively.