Replication of hepatitis A virus (HAV) in MRC-5 cells was studied under one-step growth conditions. Viral replication neither interfered detectably with cellular DNA, RNA, and protein synthesis, nor could cytopathologic changes be recorded over a prolonged period of incubation. Synthesis of mature, infectious HAV particles could be detected as early as 2-4 days p.i. and occurred at a maximal rate around 8 days p.i., shortly before infectivity titers reached a plateau. Synthesis of total viral RNA, of positive-strand genomic RNA, of viral mRNA, as well as of negative-strand RNA followed the same pattern. By Day 14 p.i., when active HAV replication had developed into persistent infection, synthesis of viral RNA declined to background levels. The mechanism(s) guiding active HAV replication into a state of persistent infection could not be positively defined. Yet there exists the possibility that this is brought about by down regulation of viral RNA synthesis. Whether this is related to the appearance of a subgenomic viral RNA molecule about 2000 nucleotides in length and detected in association with ribosomes on Days 7 and 10 p.i. remains to be shown.