The persistence of hepatic fibrosis and of Schistosoma japonicum eggs in the tissues of mice was examined after chemotherapy. C57BL/6 mice infected with a Philippine strain of S. japonicum were treated with praziquantel or amoscanate 7 or 8 weeks after infection. Groups of mice were killed at the time of treatment and 3, 8, 26, and 52 weeks later. The number of eggs per worm pair in the tissues did not change during the year after treatment. However, S. japonicum eggs injected into the tail vein were gradually destroyed in the lungs. Hepatic fibrosis increased in the 1st few weeks after treatment and did not change significantly thereafter. Praziquantel, but not amoscanate, had an immediate toxic effect on the most mature eggs in the tissues which was accompanied by a marked but transient decrease in eggs passed in the feces. Less mature eggs appeared unaffected by the drug and the passage of eggs in the feces resumed as these matured. Egg passage then ceased as the supply of viable eggs was exhausted.