The synthesis and secretion of vitellogenin by the ovary of Rhodnius prolixus was investigated. Using whole ovary or epithelial cells isolated from follicles of different sizes, it is shown that the follicle cells are a site of synthesis for this protein in the ovary. The ovaries or follicle cells were incubated in vitro with 35S-methionine or 32Pi and the secretion of newly synthesized ovarian vitellogenin (O-Vg) was estimated by the radioactivity associated with the immunoprecipitate or acid-precipitate proteins in the culture medium. The radioactive O-Vg was analyzed by SDS-PAGE followed by autoradiography or after elution from a DEAE-Toyopearl column. The presence of O-Vg inside the follicle cells was detected by immunofluorescence and immunogold labels. Both methods revealed strong labeling inside the follicle cells. While the capacity for total protein synthesis by the follicle cells was maximal during the early phase of vitellogenesis (in small follicles), the synthesis of O-Vg reached its peak during the late phase of oocyte growth, just before formation of the chorion. A possible role for ovarian vitellogenin in Rhodnius and its relationship with Vg synthesis by the fat body is discussed.