One hundred and forty mice were infected orally with 1000 embryonated Toxocara canis eggs. Groups of 10 mice were then treated with high doses of piperazine, mebendazole, oxfendazole, albendazole, fenbendazole and diethylcarbamazine for four days, either immediately after infection or three weeks after infection. The mice were killed three to six weeks after treatment and the number of larvae in the brains and muscles estimated and compared with those in untreated mice. Few larvae were recovered from the muscles of any of the mice, including the untreated controls, despite the use of a variety of recognised techniques. Large numbers of live larvae were recovered from the brains of the mice and there was no significant difference between the treated and untreated mice.