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 six (metronidazole, tinidazole, mebendazole, trichlorophone, oxamniquine 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 months 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.