Ultrastructural changes taking place in the oviduct epithelium of the magnum region were studied over a period of 0–24 hr after a single subcutaneous administration of 5 mg progesterone to immature chicks. Progesterone induced in 12 hr a synthesis of a specific protein, avidin, which declined during the following 12 hr and reached zero level at 48 hr after progesterone. The epithelium of the magnum increases in height, and several submicroscopic changes take place due to progesterone. The nuclear outline is irregular in the control oviducts and in those 24 hr after the administration of progesterone, whereas before and during the active synthesis of avidin, the nuclei are smooth. There were no changes in the nuclear size. Detachment of heterochromatin from the nuclear envelope occurs 2 hr after progesterone. There is a slight initial decrease in the amount of perichromatin granules (35–45 nm) followed by an increase exceeding the level of the controls 12 and 24 hr after progesterone. The number of nucleoli doubles within 24 hr after progesterone. Concomitantly, the granular and fibrillar part becomes dispersed. The number of polysomes in the cytoplasm increases in 2 hr. Seven hours after the administration of progesterone, the rough endoplasmic reticulum-mitochondrial apposition is prominent. The Golgi apparatus increases in size in 2 hr and begins to vesiculate in 24 hr. The mitochondria increase in number 2 and 7 hr and in size 12 and 24 hr after progesterone. There is a progressive interdigitation of the plasmamembrane beginning 2 hr after progesterone. The first secretory granules appear in 12 hr. Part of the results can be explained on the basis of transcriptional and translational changes during the progesteroneinduced differentiation of the oviductal epithelium.