In confirmation of experimental results previously obtained the investigation of the respiratory frequency of rats and hamsters exposed to inhaled cigarette smoke by means of different principles of procedure revealed that the intake of smoke is no more guaranteed if CO-intoxication exceeds a certain level. The experimental results demonstrate by comparison that smoke inhalation at fixed intervals involves a higher risk of reflex apnea. In relation to the duration of exposure the "open" inhalation procedure affects the frequency of respiration and the one-minute respiratory volume to a smaller extent. The "open" smoking technique permits the intake of larger quantities of smoke and causes less danger to the experimental animal. Hamsters proved to be considerably less susceptible to nicotine and were, therefore, found to be more suitable for inhalation experiments. It should be pointed out that the individual animal species varied in alteration of the respiratory rate. It is once again shown that inhalation experiments are able to produce good results only on the condition that a relatively small CO-intake guarantees a sufficient respiratory frequency.