The synthesis of influenza A virus RNA and proteins represents a highly regulated process whereby variable amounts of early and late viral RNAs and proteins may be produced. This regulation is upset by the presence of the methyltransferase inhibitor 3-deazaadenosine (3DA-Ado) or the protein kinase inhibitor H7, resulting in complete or partial inhibition of synthesis of late proteins but normal production of early proteins. Although the total yield of viral mRNAs is somewhat reduced by treatment with 3DA-Ado, the mRNAs that are produced can still be translated in vitro. Both 3DA-Ado and H7 interfere specifically with the transport of the late viral mRNAs from the nucleus to the cytoplasm, but do not affect transport of early mRNA. From these results we conclude that during influenza virus replication, posttranscriptional regulation takes place on the level of mRNA transport. Since hemagglutinin mRNA migrates to the cytoplasm in the presence of 3DA-Ado plus cycloheximide, we assume that a viral protein is involved in the regulation mechanism.