Selected naturally occurring flavonoids were shown to inhibit three reverse transcriptases (RT): avian myeloblastosis (AMV) RT, Rous-associated virus-2 (RAV-2) RT and Maloney murine leukemia virus (MMLV) RT when poly (rA)oligo(dT)12-18 or rabbit globin mRNA were used as template. Amentoflavone, scutellarein and quercetin were the most active compounds and their effect was concentration-dependent. The enzymes exhibited differential sensitivity to the inhibitory effects of the flavonoids. The compounds also inhibited rabbit globin mRNA-directed, MMLV RT-catalyzed DNA synthesis. Amentoflavone and scutellarein inhibited ongoing new DNA synthesis catalyzed by RAV-2 RT. Kinetic studies were performed in an attempt to elucidate the mechanism of action of amentoflavone and scutellarein. A model is proposed for the mechanism of action of the flavonoids on RT activity.