The regeneration of the islets of Langerhans in the guinea pig was studied after intravenous injection of alloxan. A number of beta cells in the islets was destroyed within 24h after alloxan, but after 48h there was a rapid proliferation of the surviving cells of the islets. This depended on the dosage of the drug as well as the timing. Electron microscopy of the islet at 48h showed that the dividing cells had small electron dense granules and resembled a sub-type of normal A cells, whose function is not yet known. There were also many agranular cells in the islet. These two groups of cells seen in the regenerating islet could be precursor cells, which could differentiate into beta cells. There was no evidence for transformation of duct cells or acinar cells into islet cells. None of the guinea pigs became permanently diabetic. This was probably due to inadequate dosage which resulted in only partial degeneration of the beta cells followed by regeneration and recovery. There was also some regeneration of the liver, kidney and the adrenal cortex following alloxan.