Mesonephric agenesis was achieved by microsurgical excision of the left Wolffian duct and the underlying intermediate mesoderm of different regions between somites 16 and 23 in chickens after 50–52 h of incubation (stage 14 HH). Quail-chick chimaeras were produced by transplantation of corresponding quail tissue in the region of somites 18–21. A morphometrical analysis of the mesonephric and gonadal area in cross sections shows that the intermediate mesoderm from somites 16 to 23 develops into the mesonephros. A partial agenesis of the mesonephros brought about by removal of the intermediate mesoderm at the level of somites 18 to 21 at stage 14 leads to a mean reduction of the gonadal volume of 37·8% compared to the volume of the untreated side at stage 30. Transplantation of quail intermediate mesoderm in this region of the excision results in development of a hybrid mesonephros. Consequently, the gonads are invaded and colonized by quail cells mobilized from mesonephric corpuscles examined at stage 30, 35 and 36. These results are discussed in terms of the origin of the gonadal stroma during this developmental period; they show that in the region from the third to the sixth segment the ventromedial part of the differentiating mesonephros participates in the contribution of stromal cells to the gonad.