Plasma concentrations of dexamethasone have been measured in sheep during an 8-day infusion of dexamethasone-21 phosphate. The dexamethasone concentration profiles generally revealed a reproducible pattern with three phases--a peak during the first 48 h infusion which was followed by falling concentrations during the next 5 days, and a small increase in dexamethasone concentration during the final 24 h infusion was not uncommon. The pattern of dexamethasone concentrations was retained when dosage was arranged in such a way as to infuse increasing quantities of hormonal analogue as infusion progressed. Aspects of the metabolism of the analogue are discussed. Endogenous thyroxine and cortisol were significantly depressed during infusion. In these experiments wool was completely shed in three out of four animals dosed at a rate of 8.5 mg dexamethasone/kg0.75. The recovery of wool growth to its pretreatment values occurred by about one month after infusion. The consumption of food and body weight increases were satisfactory during the post-infusion period.