Immature chicks were used for the experiments 1-2 days after hatching. A group of chicks were injected with 5 mg of progesterone and sacrificed 1 or 24 hr later. An other group of chicks were injected daily for 9 days with 5 mg of diethylstilbestrol and thereafter with single injection of progesterone. Cryostat sections were incubated with rabbit anti-avidin serum and with fluorescein labelled antirabbit globulin for fluorescence histochemistry. In the control animals no epithelial cells of the oviduct were fluorescence positive independently whether or not the animals were pretreated with diethylstilbestrol. One hour after administration of progesterone epithelial cells showed occasionally a slight synthesis of avidin. 24 hours after the injection of progesterone most, however never all, of the epithelial cells showed avidin in the apical part of the cell. The fluorescence reaction was clearly more intense if the animals were estrogen-primed. Diethylstilbestrol caused a differentiation of oviductal glands which were, however, only occasionally avidin positive after progesterone. These results suggest that primitive oviductal cells can produce avidin without preceding differentiation whereas estrogen causes a differentiation of new line of cells which have regularly lost their capacity of avidin synthesis after progesterone administration.