In a village hyperendemic for onchocerciasis in Burkina Faso, where the vector had been controlled by the Onchocerciasis Control Programme since 1976, 65 patients were treated with doses of 37 to 71 mg/kg suramin in 1979. The viability and fertility of the adult parasites were studied in 217 nodules excised from 42 patients one month to four years after treatment using the collagenase technique. Most of the worms had survived the treatment. Male worms were more susceptible to suramin than female worms. The development of intrauterine stages initially continued, but was completely suppressed after several months. The female worms remained sterile in those patients who had received more than 60 mg/kg suramin. In other ones 11 and 13% of the female worms showed new embryonic stages in the uteri during two follow-up examinations in the second year. In a final examination in 1983 the reproduction had declined again. However, the reproductivity of the superannuated worms had considerably decreased in the untreated patients as well. All patients had shown a very high microfilardermia in 1979. After treatment the microfilarial densities dropped to levels near zero depending on the dose of suramin administered, but in none of the patients was the reduction complete during the first year of the treatment. Microfilariae gradually reappeared in the second year in several patients who had received less than 60 mg/kg suramin. In the final follow-up in 1983 most of the treated patients had become negative in skin counts, although they still harboured viable worms.(ABSTRACT TRUNCATED AT 250 WORDS)