Summary Diversion of the portal blood flow from the liver in rats by construction of end-to-side portacaval shunts increased DNA synthesis in hepatocytes, presumably as a response to liver cell damage. Electron microscopically increased variability in staining and electron density, cytoplasmic vacuoles and mitochondrial changes were noted. The liver cell damage associated with diversion of the portal flow may account in part for some of the undesirable side effects associated with portacaval shunts in man.