Cyanide concentrations in various organs (the blood, liver, lung, spleen and brain) of the rats intoxicated by ingestion or inhalation were determined with a slightly modified microdiffusion method. In the inhalation experiment, a rat in an exposure chamber inhaled HCN produced by addition of NaCN solution to H2SO4. For each route two different doses were used. The time to death was shorter when doses were larger. Cyanide concentrations were higher in per os group than in inhalation group in all the organs tested. The concentration in the liver was most sensitive to the route of administration; it was the highest among the organs tested after per os administration, but low after inhalation. In the lung it was significantly higher than that of the blood when cyanide was inhaled. It was concluded that, when it is necessary to determine whether cyanide was ingested to inhaled, at least the lung and liver must be analyzed for cyanide contents in addition to the blood and stomach content.