Abstract: 1. A methed for estimation of phenelzine, based on the use of trichloroacetic acid to precipitate the protein of plasma and tissues, and the use of dimethylaminobenzaldehyde as a colour developing agent, was found to be adequate for our purpose. 2. When phenelzine was given to a mouse orally or intraperitoneally, the peak plasma level was obtained in 0.5—1 hour, which then declined gradually. The compound was distributed in the liver, brain, heart, kidney, and lung. The highest concentrations were attained for the first two organs, while the concentration of phenelzine for the lung was very low. The rate of disappearance of phenelzine from tissues was generally found to be slower than from the plasma. 3.In mice, the LD₅₀ of phenelzine for single dose of intraperitoneal injection was 151.4 mg/kg, and that for a series of 9 daily doses was 45—50 mg/kg.In rabbits, the lethal dose of phenelzine was 75—100mg/kg. 4. In rabbit, intravenous administration of 2, 5, 10 mg/kg of phenelzine stimulated the respiration, promptly abolished respiratory depression caused by morphine and slightly elevated the blood pressure. A dose of 25 mg/kg slowed the heart rate. Atropine partially blocked this action. After given 10 mg/kg daily for a week, the plasma retention of bromosulphthalein may be elevated, and the fatty degeneration of liver tissues developed. In mice, intraperitoneal administration of 0.4 mg/kg, of phenelzine increased the anaesthetic time of the evipan. Phenelzine was not an antipyretic.