In order to analyze the intracellu1ar localization of specific RNA components of ribonucleoproteins (RNP) in Xenopus oocytes, a modified protocol of whole-mount in situ Hybridization is presented in this paper, Mitochondria specific 12S rRNA probe was used to detect the amplification and distribution of mitochondria in various stages of the oocyte life cycle, and the results were found to be consistent with previously known distribution of mitochondria. The results with other specific probes (U1 and U3 small nuclear RNAs, and 5S RNA) also indicate that this procedure is generally effective in localizing RNAs in RNP complexes even inside organelles. In addition, the RNA component of RNase MRP, the RNP with endoribo-nuclease activity, localize to the nucleus in various stages of the oocyte life cycle. Some of MRP RNA, however, were found to be localized to the special population of mitochondria near the nucleus, especially in the active stage of mitochondrial amplification. It suggests dual localization of RNase MRP in the nucleus and mitochondria, which is consistent with the proposed roles of RNase MRP in mitochondrial DNA replication and in rRNA processing in the nucleolus.