Responses to several drugs called cataleptogenic as shown in the "Catalepsy Testbattery", were analyzed for their trigger mechanism. The results lead to the conclusion that the neurophysiological mechanisms of neuroleptic drugs are based on an inhibition of the processing of afferences. Therefore the term "experimental catalepsy" seems to be applicable only to these drugs. On the contrary, opioid drugs induce a state of rigidity which even can be increased by manipulating the animal. These conclusions are verified by electromyographical studies. In our opinion this syndrome should not be called "catalepsy" but "opiate stiffness".