MUCH work has already been carried out on the treatment of mice infected with schistosomes. It has been observed that the worms which normally inhabit the mesenteric veins of animals infected with Schistosoma mansoni are forced to migrate to the liver after treatment with active schistosomicidal drugs1–6. This hepatic shift has usually been observed after a full course of treatment, and where the course had been inadequate, relapses were found to occur owing to the re-migration of the worms back to the mesenteric veins. Standen6observed a hepatic shift as early as one day following two oral doses of 42 mgm./ kgm. sodium antimonyl III gluconate, but after 7 days the worms were completely restored to their normal distribution in the hepatic portal system. In the work reported here we followed up the effect of a single dose of tartar emetic on the worm migration to show that the hepatic shift actually occurs much earlier than had been previously expected