Cases of onchocerciasis were treated with three drugs, Antrypol, Pentostam and Hetrazan. Antrypol proved undoubtedly effective, producing disappearance of the microfilariae and onchocercal nodules, and improvement in cases of defective vision with anterior ocular manifestations, but little or no improvement in cases with well-defined posterior manifestations. The severe toxic effects of the drug, however, are sometimes fatal, and therefore preclude its use. Pentostam, although non-toxic, did not prove effective in the doses tried; it may be of some use in very large doses. Hetrazan was given to three cases only, in all of which it produced very rapid disappearance of the microfilariae, but this was accompanied by some allergic reactions and there was no other indication of improvement. Moreover, one month after treatment, microfilariae reappeared in one case. It seems that the more actively filaricidal a drug is, the more toxic it will be, and this is likely to be a serious limitation in the chemotherapy of onchocerciasis until some method of avoiding the allergic reactions can be devised.