SYNOPSIS. Three yolk polypeptides (YPs) are major constituents of eggs. YPs are synthesized in the fat body and in ovary associated cells, secreted into the blood and sequestered into developing oocytes. YPs are translated as precursors which can be processed to lower molecular weight polypeptides by dog pancreatic microsomes, suggesting signal peptide removal. YP synthesis in both ovary and fat body is stimulated by juvenile hormone (JH), but 20-hydroxyecdysone (20HE) only stimulates YP synthesis in fat body. JH also induces YP sequestration into oocytes. The amount of RNA translatable into YPs increases sevenfold in the first day after eclosion. The increase can be blocked by ligating the abdomen to isolate it from anterior endocrine organs, and it can be restored by injecting these preparations with 20HE. Using electrophoretic variants, the YPs have been genetically mapped to the X chromosome:Yp1 and Yp2 are adjacent and Yp3 is distantly linked. Two mutants have been described which decrease the quantity of a single YP, map near the respective YP structural loci, are cis-acting and are not ovary autonomous in transplants. fs(1)1163 alters the structure of the primary translation product and is hypothesized to alter YP processing and secretion. Although Yp3RI results in no detectable YP3, the mutant genome and mutant RNA contains sequences complimentary to cloned Yp3 gene. This mutant may result in blocked translation. Further analysis of Drosophila vitellogenesis using molecular and classical genetic techniques promises to help us understand how hormones regulate gene activity.