The article describes a technique for forming molecular hybrids between RNA in solution and the DNA of intact cytological preparations. The method involves immobilizing cells in a conventional tissue squash under a thin layer of agar, denaturing the DNA with alkali, and incubating with tritium-labeled RNA. The hybrids are detected by autoradiography. The technique is illustrated using ribosomal RNA (rRNA) hybridization to amplified ribosomal genes in oocytes of the toad Xenopus. The authors also detect a low level of gene amplification in premeiotic nuclei (oogonia). The procedure is detailed, including the preparation of the slides, denaturation, hybridization, and detection steps. The results show that the technique can detect rDNA amplification in early meiotic stages, with the label localized to the nucleoli. The sensitivity of the method is discussed, and potential improvements, such as using in vitro synthesized RNA, are suggested.The article describes a technique for forming molecular hybrids between RNA in solution and the DNA of intact cytological preparations. The method involves immobilizing cells in a conventional tissue squash under a thin layer of agar, denaturing the DNA with alkali, and incubating with tritium-labeled RNA. The hybrids are detected by autoradiography. The technique is illustrated using ribosomal RNA (rRNA) hybridization to amplified ribosomal genes in oocytes of the toad Xenopus. The authors also detect a low level of gene amplification in premeiotic nuclei (oogonia). The procedure is detailed, including the preparation of the slides, denaturation, hybridization, and detection steps. The results show that the technique can detect rDNA amplification in early meiotic stages, with the label localized to the nucleoli. The sensitivity of the method is discussed, and potential improvements, such as using in vitro synthesized RNA, are suggested.