It was reported earlier that the biosynthesis of small nuclear RNAs (snRNAs) (U1, U2, U3, U4, and U5) shows an unexpected great sensitivity to ultraviolet (UV) radiation (254 nm). In this "early" inhibition, snRNA formation is suppressed immediately after exposure to UV light. There is also a second "late" inhibition of snRNA biosynthesis which requires lower doses of UV radiation and 1-2 h of postirradiation cell incubation to develop fully. In the present work we asked which step, within the metabolic pathway leading to the accumulation of newly made snRNA, is affected by UV light. Both for the early and late UV radiation-induced inhibitions: (a) similar results were obtained after pulse labeling or pulse chasing the radiolabel, implying that UV light did not decrease the stability of newly made snRNA; and (b) gel electrophoretic analysis of radiolabeled RNA that had been hybrid selected with cloned snRNA genes showed no accumulation of putative snRNA precursors, suggesting that UV radiation did not block snRNA processing. Instead, when transcription was carried out in isolated nuclei from irradiated cells, the effects of "early" and "late" inhibition were reproduced, indicating that transcription was affected. The early suppression appears to be a separate reaction from the late inhibition, since U1 snRNA transcription in isolated nuclei was inhibited in the absence of postirradiation cell incubation. There is a small fraction of snRNA synthesis that is resistant to high UV light doses (greater than or equal to 870 J/m2) right after irradiation, but is sensitive to lower doses (less than or equal to 36 J/m2) when the cells are incubated for 2 h after irradiation.