There are few papers on the biosynthesis of serotonin in the enterochromaffin cell in the pancrease. The present studies were performed to clarify this problem by use of autoradiography in the auinea pig injected of 5-HTP-H3 in dose of 1 vc/g. body weight, The appearance of the above mentioned cells in the pancreas of the rabbit was also studied morphologically. The results are as follows ; 1. Enterochromaffin cells were found in the epithelia of all the pancreating ducts and a few acini of normal male rabbit and most numerous in the intramural pancreatic ducts. 2. The redioautographic silver grains overling the enterochormaffin cells in the guinea pig pancreas are considered to be due to the newly synthesized radioactive serotonin in the cells from injected 5-HTP-H3.