1. The administration of dexamethasone to intact fed rats by rats by intraperitoneal injection for 3 h was associated with a 6-fold increase in the time for which mitochondria subsequently isolated from the liver retain a given load of exogenous Ca2+. This effect was blocked by co-administration of cycloheximide with dexamethasone, and partially blocked by the co-administration of puromycin. Daily administration of dexamethasone for periods of 4-7 days resulted in liver mitochondria that exhibited a decreased ability to retain exogenous Ca2+. 2. When glucagon was administered to fed adrenalectomized rats, the increase in mitochondrial Ca2+ -retention time that results from the action of this hormone was reduced by 50% when compared with its effect on intact animals. The administration of dexamethasone to adrenalectomized rats partially restored the full effect of glucagon. 3. Dexamethasone did not enhance the effect of glucagon on mitochondrial Ca2+ -retention time when administered to intact fed rats. 4. It is concluded that these data support the hypothesis that the hormone-induced modification of liver mitochondria, which results in an increase in the time for which exogneous Ca2+ is retained, involves a step in which new protein is synthesized.