The aim of the study was to investigate acute (96-h LC50) toxicity of the complex metal (Zn, Pb, Cd, Cu, Ni and Cr) mixture (MIX) to European perch (Perca fluviatilis) and rainbow trout (Oncorhynchus mykiss), and to examine differences between locomotor and respiratory endpoints based on rapidness and sensitivity of fish responses to short-term (2 h) metal mixture exposure. MIX was prepared taking into consideration the maximum-permissible-concentrations (MPC) set for these metals in EU inland waters (Directive 2008/105/EC). The studied behavioral and respiratory responses of O. mykiss, in contrast to those of P. fluviatilis, were found to be significantly affected by the relationship between exposure duration and treatment. In O. mykiss, gill ventilation frequency (GVF) was found to be the most rapid and sensitive endpoint of all the investigated ones. However, the investigated behavioral and respiratory endpoints of P. fluviatilis showed its behavioral insensitivity to MIX exposure.