This invention relates to ornithine derivatives substituted at the 5 position with an R group which can be a --CH.sub.2 F, --CHF.sub.2, --CHClF, --C.tbd.CH, --CH.dbd.CH.sub.2, or --CH.dbd.C.dbd.CH.sub.2 group and the pharmaceutically acceptable acid addition salts thereof which are specific inhibitors of OAT and can be used in the treatment of conditional deficiencies of ornithine.