Hyperplasia of the islands of Langerhans involving the a cells was observed in the pancreatic remnant of partially depancreatized rats. There was no correlation between the degree of hyperplasia and the degree of impairment of sugar tolerance. The anatomic changes observed are similar to those reported by other workers following the inj. of ant. pituitary extracts. The difference in the reaction of the residual islet tissue of the dog and of the rat to partial pancreatectomy is pointed out.