Metrifonate has been studied in the treatment of infections with Onchocerca volvulus in West Africa. In doses of 10 mg/kg daily for six days, metrifonate was effective against the microfilariae but produced unpleasant reactions due both to the death of the microfilariae and to the muscarinic effects of the drug. In a comparison of 10 mg/kg metrifonate, for 3 doses at 10 day intervals, with diethylcarbamazine (D.E.C.) in a total dose of 6.6 g, metrifonate was significantly less effective than D.E.C. in killing microfilariae but produced significantly fewer adverse effects. In a final study comparing metrifonate 10 mg/kg/day given for 3 or for 6 days there was little difference in their therapeutic effect. However, metrifonate 10 mg/kg given for 6 days produced more severe adverse reactions due largely to the muscarinic and nicotinic effects of acetylcholine and these effects were not prevented by the administration of belladonna alkaloids. Metrifonate 10 mg/kg given for 3 doses at 10 day intervals is effective in the treatment of onchocerciasis but is unlikely to replace diethylcarbamazine. Metrifonate has no action on the adult worms.