In the tail-flick test as well as in the late phase in the formalin test skin temperature may in an important way influence the response. A reduced skin temperature may be misinterpreted as analgesia, and an increased skin temperature as hyperalgesia. These effects and the mechanisms that cause them are discussed. It is of particular importance to be aware of these confounding factors when using drugs or making lesions that influence blood flow or temperature regulation. It is important that all variables influencing the test results are kept constant throughout the experiment. This includes not only the ambient temperature, but also factors that may influence the vasomotor tone of the animals.