This study compiles observations on the reproductive capacity of O. volvulus. Adult parasites enzymatically isolated from excised onchocercomata of untreated and chemotherapeutically treated patients, and from inhabitants living in areas with long vector control, were investigated to assess their fecundity. Changes of microfilaria development in utero and microfilaria release were assessed or estimated after treatment of patients with micro-filaricidal drugs that interfered with the development of intra-uterine stages. Intra-uterine production of microfilariae: After treatment of patients with ivermectin a daily development of 2500 to 4000 uterine microfilariae per female worm was observed. Actual output of microfilariae: Microfilariae left actively the female worms. The daily microfilaria release in vivo was 700 to 900 microfilariae, assessed after treatment of patients with mebendazole. In vitro most worms isolated from untreated patients shed between 500 to 1500 microfilariae per day.-The microfilarial load of 56 adult patients calculated from microfilarial skin counts was 12 million on the average. Taken for granted a mean life span of a microfilaria of 1.0 to 1.5 years, 22 to 47 female worms per patient would suffice to maintain this microfilarial load on a constant level. Excision of all palpable nodules showed a geometric mean of 15.9 female worms in these patients. It is suggested that factors intrinsic in the host and the adult worms partially operate together to regulate and maintain a stable microfilarial density.