The cytoplasmic antigen and ultrastructural changes we described previously for chimpanzees (Pan troglodytes) infected with hepatitis C virus (HCV) or with hepatitis D virus have recently been shown to be indirect measures of viral replication and appear to represent a host response to the expression or action of interferon. The time of appearance of these changes in hepatocytes during HCV infection, when compared with similar changes in hepatitis D virus infection, suggests a very early replicative phase for HCV. To investigate the early events in HCV infection, we infected two chimpanzees with HCV and obtained blood and liver biopsy samples from them daily during the first 10 days of infection. The early stage of infection with regard to HCV replication, antigen expression, and ultrastructural changes was similar in both chimpanzees. When tested by cDNA/polymerase chain reaction, HCV sequences became detectable in the serum as early as 3 days after inoculation and remained positive through the peak of aminotransferase elevations. In one chimpanzee the peak of virus production appeared to be 7 weeks after inoculation, which was coincident with rising enzyme values. The cytoplasmic antigen, detected by immunofluorescence, and ultrastructural changes, detected by electron microscopy, became positive in hepatocytes 3 and 6 days, respectively, after HCV sequences were first detected in serum. Circulating anti-HCV appeared 13 weeks and 32 weeks after inoculation, respectively, in the chimpanzees. These data indicate a very early replicative phase for HCV and a potentially long period of infectivity before the appearance of anti-HCV.