In D. viteae infected M. natalensis oral or subcutaneous (s.c.) treatment with ivermectin on 5 consecutive days with at least 0.05 mg/kg and single dose treatment with 0.1 mg/kg caused a 100% reduction of microfilaraemia throughout the investigation period of 42 days. Using lower doses (lowest dose used 5 X 0.003 mg/kg) animals were free from circulating microfilariae at least until day 7. The drug was active against adult worms. Treatment with doses of 5 X 0.2 mg/kg and above resulted in 81-93% and 38-83% reductions of male and female worms, respectively. Lower doses caused inconsistent macrofilaricidal effects but mainly male worms were affected. After treatment with 5 X 3.25 mg/kg and above increased rates of pathologically altered intrauterine stages were found in surviving female worms. After treatment of L. carinii infected Mastomys with doses of at least 5 X 1.5 mg/kg animals remained amicrofilaraemic until autopsy on day 42 and in case of lower doses (lowest dose used 5 X 0.006 mg/kg) at least until day 7. After single dose treatment (s.c.) with 0.2 mg/kg and above animals were free from microfilariae in the blood throughout the observation period. Single dose treatment with 0.05 and 0.0125 mg/kg removed all microfilariae from the blood until 8 and 48 hours, respectively but microfilariae occurred in the blood again after 14 days. Ivermectin did not affect the numbers of adult L. carinii but female worms isolated 42 days after treatment with 5 X 0.78 mg/kg and above were free from motile, normally shaped microfilariae and contained increased rates of pathologically altered embryonic stages.(ABSTRACT TRUNCATED AT 250 WORDS)