Summary In the study of the effect of oral administration of quinacrine hydrochloride and chloroquine phosphate on Opisthorchis infection, 65 patients were treated in Northeast Thailand. Quinacrine in the amounts used and over the periods tested failed to have any appreciable effect on the infection. On the other hand, among the patients treated with chloroquine, a marked reduction in the egg laying capacity of the worms was frequently observed as a result of treatment. In 30 out of 32 patients treated with chloroquine for 16 consecutive days or longer there was a decrease or a disappearance of Opisthorchis eggs in the stools. Observations of 23 patients up to 90 days after treatment was discontinued seem to indicate that, in most cases, the reduction in egg production is not just a temporary phenomenon due to stunning or to transient sterilization of the adult flukes. Significant toxicity was not observed during this study.