The synthesis of an endogenous ribosomal protein, L1, is selectively and efficiently inhibited by microinjection of antisense L1 RNAs into Xenopus oocytes. Repression of L1 synthesis is achieved within 12 hr and is maintained for 48 hr. RNase-protection assays reveal the formation of RNA X RNA duplexes in vivo between the endogenous L1 mRNA and injected antisense transcripts. Partial-length antisense RNAs, complementary to only the 3'-terminal region of L1 mRNA, repress translation as effectively as a full-length antisense RNA, indicating that complementarity to the 5' region of L1 mRNA is not required for efficient inhibition. The use of antisense RNA to repress synthesis of an endogenous ribosomal protein provides a functional basis for determining mechanisms that integrate ribosomal protein synthesis with ribosome assembly during oogenesis.