Synthesis of vitellogenin (yolk protein) by the fat body of Aedes aegypti is triggered by the blood meal. Total RNA of the fat body begins to rise 2 hr post blood meal (PBM) and increases 3-fold by 12 hr. Vitellogenin synthesis is detectable 3-4 hr PBM, and reaches a peak by 28 hr PBM. After 28 hr PBM both total RNA and the ability to synthesize vitellogenins fall precipitously. Actinomycin D at 10 μg/ml inhibits RNA synthesis by about 90%, but does not inhibit in vitro synthesis of tissue proteins. At this concentration of actinomycin, vitellogenin synthesis remains constant in vitro for up to 6 hr, suggesting the presence of a relatively long-lived messenger RNA. When injected into mosquitoes, actinomycin prevents the normal increase in the rate of vitellogenin synthesis but allows synthesis to proceed at the rate occurring at the time of injection. The results suggest that the blood meal triggers the synthesis of both messenger and ribosomal RNA necessary for later vitellogenin synthesis.