In contrast to procaryotes, eucaryotic cells contain a discrete set of low molecular weight RNA species with a size distribution between 4 S and 7 S (Dingman and Peacock, 1968; Weinberg and Penman, 1968; Hodnett and Busch, 1968). Studies using cell fractionation procedures demonstrated the existence of a cytoplasmic group (small cytoplasmic RNA, scRNA) and a nuclear group (small nuclear RNA, snRNA) among these low molecular weight RNA species (Zieve and Penman, 1976). As to the biosynthesis of small RNA molecules it has been shown that, in addition to 5 S and transfer RNA, scRNA species too are synthesized by RNA polymerase C or III (Zieve et al. 1977), whereas nothing is known about the transcription of snRNA. The exact biological function of the small molecular weight RNA species remains widely obscure at present. However, snRNA molecules have been described as part of the nuclear ribonucleoprotein particles (RNP) carrying heterogenous nuclear RNA (hnRNA) (Deimel et al. 1977; Prusse et al., submitted; Zieve and Penman, submitted). Two recent reports, one describing sequence homologies between snRNA species and the splicing points of hnRNA (Lerner et al. 1979) and the other describing a coordinate regulation of snRNA synthesis and messenger RNA biogenesis (Benecke et al. 1980), point to a possible involvement of small nuclear RNA species in hnRNA metabolism.