Following oral administration of 3H-2-(7-methoxy-10-methyl-2-phenothiazinyl)-propionic acid (PRT) to rabbits and dogs, radioactivity level in blood increased rapidly to a maximum at 1 hr. In the rabbits, 87% of the administered radioactivity was excreted in the urine, whereas 73% of the administered radioactivity was excreted in the feces in the dogs within 72 hr after dosing. In pregnant rats given 3H-PRT orally, radio-activity in thefetuses did not exceed 1% of the administered radioactivity. When 3H-PRT was given orally to rats having carrageenin-induced edema in one of their hind feet, radioactivity level in the inflamed foot was found to be significantly higher than that in the untreated foot. Aminopyrine demethylase and aniline hydroxylase activities of liver microsome of rats were determined after oral administration of PRT for 4 days and for 30 days, respectively. No significant difference was found in the enzyme activities between the rats given PRT and control animals.