The pattern of gene expression for specific genes during oogenesis has been examined by in situ hybridization to RNA in sectioned ovarian preparations. The accumulation (1) of RNA complementary to heat-shock genes, both before and after heat shock; (2) of RNA complementary to three lambda phage recombinants which contain sequences expressed during oogenesis; and (3) of RNA complementary to the rudimentary gene have been analyzed. Gene expression in the ovaries is found to be dependent on both the cell type and the developmental stage of the ovarian chamber. In the nurse cells, the onset of vitellogenesis (stages 8-11) appears to be accompanied by a substantial activation of many different genes and there is a pronounced increase in the level of accumulation of complementary RNAs. Moreover, much of the RNA synthesized in nurse cells during vitellogenesis is ultimately transported into the growing oocyte where it accumulates. It is likely that this accumulation of RNA provides a source of gene products, either as message or proteins, for embryogenesis.