The activity of diethylcarbamazine (DEC) and suramin (S) on Litomosoides carinii filariasis was assessed by the study of microfilaraemia in 30 cotton-rats (Sigmodon hispidus). DEC (per os) and S (injected subcutaneously) were administered alone or in combination, at doses of 50 mg/kg/day for five consecutive days. 15 rats were treated at day 91 after infection and 15 others at day 184. The immediate but temporary efficacy observed with DEC was in contrast to the delayed and long-lasting effect of S. With combined therapy there was neither antagonism nor synergism. The demonstration of an apparently smaller reduction of parasitaemia in the rats treated with DEC before the plateau phase of the microfilaraemia was consistent with the lack of action of this drug on microfilariae outside the blood system.