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The ribonucleoprotein enzyme RNase P catalyzes the 5′ processing of pre-transfer RNA, and has also recently been implicated in pre-ribosomal RNA processing. In the present investigation, in situ hybridization revealed that RNase P RNA is present throughout the nucleus of mammalian cells. However, rhodamine-labeled human RNase P RNA microinjected into the nucleus of rat kidney (NRK) epithelial cells or human (HeLa) cells initially localized in nucleoli, and subsequently became more evenly distributed throughout the nucleus, similar to the steadystate distribution of endogenous RNase P RNA. Parallel microinjection and immunocytochemical experiments revealed that initially nucleus-microinjected RNase P RNA localized specifically in the dense fibrillar component of the nucleolus, the site of pre-rRNA processing. A mutant RNase P RNA lacking the To antigen binding domain (nucleotides 25–75) did not localize in nucleoli after nuclear microinjection. In contrast, a truncated RNase P RNA containing the To binding domain but lacking nucleotides 89–341 became rapidly localized in nucleoli following nuclear microinjection. However, unlike the full-length RNase P RNA, this 3′ truncated RNA remained stably associated with the nucleoli and did not translocate to the nucleoplasm. These results suggest a nucleolar phase in the maturation, ribonucleoprotein assembly or function of RNase P RNA, mediated at least in part by the nucleolar To antigen. These and other recent findings raise the intriguing possibility of a bifunctional role of RNase P in the nucleus: catalyzing pre-ribosomal RNA processing in the nucleolus and pre-transfer RNA processing in the nucleoplasm.