In the use of any drug there always arise problems of its effect and side-effect as well as its toxicity and distribution in vivo. Therefore, the author studied these problems on PAM from various angles and obtained the following results. 1. The median lethal dosage (LD(50)) of PAM on mice has been found to be: 159 mg/kg in the case of intravenous administration; 247 mg/kg in peritoneal injection; 340 mg/kg in subcutaneous injection; and 1960 mg/kg in oral administration, proving that the limit is sufficiently within the range of commonly used drugs. 2. Histological changes occurring after PAM adminstration are consisted mainly of disturbances in the circulatory system, namely, hyperemia, congestion and slight hemorrhage, but rarely of parenchymal disturbances. 3. When a large amount of PAM is administered to rabbits, there occurs a rise in the blood cholinesterase activity at first later followed by its fall. However, the fall being down to -25 per cent, it can not be considered as due to the toxicity of PAM. 4. In the oral adminstration of PAM to rabbits, PAM can be detected considerably in stomach, intestines, and in their contents, but it is distributed fairly evenly in other viscera. Moreover, in the case of the intravenous administration of PAM is also distributed in similar manner. It diminishes relatively promptly from the body fluid. Judging from these findings as well as from results of clinical observations, there is no danger of toxicity of PAM in the amount as used in our clinic.