The secretory pathway of the protein was chased by electron microscope autoradiography in the fat body cells of the pupa immediately before adult in Pieris rapae L. In the fat body cells exposed in vitro for five min. to [$^3$H] leucine, the rough endoplasmic reticulum and the condensing vaculoes were radioactively labelled. The protein granules which were elaborated from condensing vaculoes were also radioactively labelled in the fat body cells exposed in vitro for ten min. to [$^3$H] leucine. Therefore, the secretory pathway of the protein in the fat body cells was initiated by protein synthesis in the rough endoplasmic reticulum, followed by the formation of condesing vaculoes from rough endoplasmic reticulum, and completed by the formation of the protein granules and then the discharge of protein granules to the cell exterior.