1. Extensors and flexors trochanteris of the second thoracic leg of Periplaneta americana were investigated physiologically and toxicologically. 2. The movements of the separate muscles were recorded with the aid of a special myographic technique. 3. Nerve muscle preparations of a completely fast and a nearly completely slow function type could be studied in this way. Some muscles represent a form in which both function types occur, probably mixed. When analysed, ryanodine appeared to be a valuable expedient. In some types of nerve muscle preparations inhibition could be demonstrated. 4. Linking up with what is known, it is reasonably certain that the action of high dosages of DDT actually takes place on motor axons or myoneural junctions and not on the muscle fibre itself. Not all of the different nerve muscle preparations seemed to be of the same sensitivity to this poison. 5. γ-HCH appears to have a very slight influence on the function of the different types of isolated nerve muscle preparations. However, because of the intense motor activity the muscles become greatly fatigued.