Ivermectin, at the standard dose of 150 cLg/kg bodyweight, does not kill the adult worms of Onchocerca volvulus and does not disrupt embryogenesis or spermatogenesis. Repeated standard doses, if maintained, arrest microfilarial production but result in only a mild-to-modest macrofilaricidal effect. We investigated whether high doses would effectively kill the adult worms, and whether cessation of microfilarial production could be reproduced by an equivalent, single, high dose. One hundred men participated in a double-blind placebo-controlled trial and received increasing doses of ivermectin from 150 &kg to 1600 &kg bodyweight. Nodules were excised at day 180 and examined by histopathology. Total doses of ivermectin up to 1600 Clg/kg were not significantly more effective than 150 &kg. Moreover, they did not reproduce the marked inhibitory effects of the repeat standard-dose regimens on embryogenesis, nor the modest effect on adult worm viability, at comparable total doses. These effects may be functions of multiplicities of dosages rather than of the total dose. Our findings also suggest that repeated high-dose regimens are unlikely to be more effective than a similar number of 150 ualka doses. This deticiencv of ivermectin reauires that the search for macrofilaricides remains a top prio& -