Hydrocortisone acetate injected into pseudopregnant rabbits induced casein synthesis and a parallel accumulation of casein mRNA. These effects were not accompanied by any enrichment of total RNA in the mammary cell. Hydrocortisone acetate did not favour the attachment of polysomes to endoplasmic reticulum. Casein mRNA concentration was enhanced in free and membrane-bound polysomes. After long treatments, the concentration of casein mRNA reached a plateau in membrane bound polysomes whereas it continued to be accumulated in free polysomes, suggesting that a substantial part of casein synthesis is then carried out by free polysomes. Progesterone injected with high doses of prolactin was unable to prevent the stimulatory action of prolactin on the synthesis of casein, the accumulation of casein mRNA and mammary gland growth, as judged by DNA content. By contrast, the increase in the total RNA content of mammary gland was still significantly reduced by progesterone. In addition, progesterone inhibited almost completely the formation of membrane-bound polysomes and the anchorage of casein mRNA to endoplasmic reticulum. From these data, it was concluded that the formation of the endoplasmic reticulum is not a prerequisite for the initiation of casein synthesis. Glucocorticoids do not play a major role in the formation of the endoplasmic reticulum and the Golai apparatus and in the binding of casein synthesizing polysomes to membranes. Progesteronne is capable of inhibiting preferentially and gradually the stimulation of cellular functions requiring the most potent prolactin stimulation.