For measurement of ciliary activity and toxic inhalation effects it is necessary to take at least the following into consideration. Comparable effects on human beings are the primary concern. Concerning choice of animal, it is doubtful whether or not results from lower species (mussels, oysters) can be directly applied to vertebrates. Few reports in which in vivo or in vitro experiments can be compared have been published. Concerning the ciliary activity and effects upon the same, there are some indications for obtaining the same results with in vivo or in vitro preparations from the same species. Observation techniques are all very similar with observation through a microscope with vertical light. For choice of registration techniques three methods are available; filming, stroboscopy, or photoelectric cell. At present, for in vivo preparations the filming is superior.