Different approaches to description of rheologically complex media in nonmonotonous mechanical processes are considered. The approach, using the common way of taking into account mechanical nonlinearity by complicated time conversion, is most thoroughly studied. The experimental data are determined while studying different nonmonotonous loading conditions for fluoropolymer film specimens. The experimental results and computer calculations of the constitutive relations are analyzed. The conclusion is made that there is no approach that can satisfactorily describe the processes under study.