The uptake of RHBP (Rhodnius heme-binding protein) by the ovaries of Rhodnius prolixus was characterized. RHBP purified from occyte was labeled with 125I and used to study the process of uptake by the ovary in vivo and in vitro. After injection, the [125I]RHBP was readily removed from the hemolymph and accumulated especially in the ovary. The capacity of the ovary to take up [125I]RHBP from the hemolymph varied during the days following blood meal. It increased up to day 2, remained stable until day 5, and then decreased up to the end of oogenesis. In vitro, the uptake of [125I]RHBP was linear at least up to 60 min. The uptake was dependent on [125I]RHBP concentration and showed to be a saturable process. The addition of a molar excess of non-related proteins such as Vitellin (Vt), Lipophorin (Lp), and Bovine Serum Albumin (BSA) did not reduce [125I]RHBP uptake. Using immunogold technique the RHBP was localized at the microvilli, coated pits, and yolk granules. The main yolk protein, Vt, did not compete with RHBP for the uptake. Thus, it is discussed here that they bind to independent binding sites of the oocytes, and are directed later on to the same compartment. The need of both proteins for the completion of mature oocyte was verified in vivo. The reduction of heme-RHBP in the hemolymph, by changing the diet, decreased the number of eggs laid. Increasing the concentration of heme-RHBP in the hemolymph, the number of eggs produced increased in a dose dependent manner. In vitro, both apo-RHBP and heme-RHBP can be taken up by the oocyte. Since the mature oocyte contains only heme-saturated RHBP, the possible fate of apo-RHBP is also discussed.