The first reports of the effects of the administration of phenothiazine to poultry appeared in 1940 and were concerned with its vermifugal activity. Since that time one group of workers published a series of articles on the effects of phenothiazine on the uptake of I131 by the thyroid gland of various animal species including poultry but the remainder of the reported in vivo studies on phenothiazine in poultry have been limited to its use as an anthelmintic against Heterakis gallinae and Ascaridia galli, Capillaria spp., and its use in the prevention and treatment of Syngamus trachea infections. Avian coccidiosis was also reported to have been cured, and outbreaks of the disease controlled by the prophylactic and curative use of the drug; in powder form it was found to be useful in the control of louse infestations of chickens. Determinations of the drug's toxicity under field conditions were also undertaken. The experiments reported herein were conducted in order to study the problem of the metabolism in chickens of purified forms of this drug.