Six drugs in common use for the treatment of parasitic infections of man were given to 18 adult patients suffering from onchocerciasis. None of the 6 (metronidazole, tinidazole, mebendazole, trichlorophone, oxaminiquine and pyrantel pamoate) showed any evidence of substantial activity against the microfilariae or adult worms of O. volvulus. The mean reduction in skin microfilarial counts a week after drug treatment (a measure of microfilaricidal action) was highest in patients treated with trichlorophone (47.0%) and mebendazole (40.0%). The rate of build-up of microfilariae over a follow-up period of 24 mo. after treatment with the drug under test followed by DEC ( a measure of macrofilaricidal action) was slowest in the groups treated with metronidazole and trichlorophone (22.9% and 27.0% of the pre-treatment counts respectively). These results fall short of those expected of drugs with potential value in the treatment of onchocerciasis.