A single intramuscular dose of HCG (50 i.u. or more per rat) was able to induce fetal resorption and eventual termination of pregnancy when injected on Day 4 or between Days 7 and 11 of pregnancy. This dose was inactive when administered on Day 12 of pregnancy. A single large dose (500 i.u./rat) induced fetal resorption when administered even on Day 12 of pregnancy. In intact rats treated with HCG, daily doses of progesterone were unable to maintain normal implantations; in ovariectomized rats treated with HCG, daily proges. terone did succeed in maintaining pregnancy in many of the animals. It is suggested that fetal resorption and the eventual pregnancy-terminating effects of HCG in rats are mediated through the alteration of normal ovarian steroidogenesis.