The present invention provides a non-human mammal, or a part of its living body, which stably retains a DNA encoding a heterologous PPAR.alpha. in an expressible state, and has one or more different genetic modifications resulting in a pathological condition identical or similar to a disease associated with the regulation of PPAR.alpha. activity or a foreign DNA under the control of a promoter having PPRE, as well as a method of screening for agonists/antagonists for the heterologous PPAR.alpha. using the animal.