This invention provides compounds that are useful for treating patients having a TGF-.beta.-mediated disease, particularly an ALK5-mediated disease. The compounds are represented by formula I: ##STR00001## wherein: a-b is CH.sub.2CH.sub.2, CH.sub.2CH.sub.2CH.sub.2, CH.dbd.CH, CH.dbd.N, or N.dbd.CH; Z is N or C--F; and G is C.sub.1-6 aliphatic or a phenyl, naphthyl, or 5-6 membered heteroaryl ring.