Sixty-three experimental and 58 control cats were infected with Brugia malayi so that the developing and adult worms localized in the regional lymphatics of the hind legs. At 20 days after infection when Brugia were in the 4th larval stage, and at 8 weeks when worms were young adults, cats were divided into groups to test the efficacy of diethylcarbamazine citrate (DEC) at various dosage levels. At 100 mg total DEC/kg no 4th-stage larvae were seen in 5 cats compared with a mean of 20.4 living larvae in each of 5 controls. At this level of DEC, 2 of 5 cats had 1 and 2 adult worms while 4 of 4 controls had a mean of 23.2 living adult worms. At 50 and 25 mg/kg there was a substantial reduction of both 4th stage and adult worms when compared to controls. At 10 mg/kg, 4 of 6 cats had 4th-stage larvae but at a lower level (mean = 7.0) than in 6 controls (mean = 23.2). No reduction of either 4th-stage larvae or adult worms was seen at 1 mg/kg. This study establishes the efficacy of DEC against 4th-stage and adult Brugia malayi in cats, although considerably higher levels of the drug were required than the level previously determined to kill 3rd-stage larvae. It appears that the cat-B. malayi model will be an effective method to compare the efficacy of drugs against adult lymphatic-dwelling filariae.