The effects of ultraviolet irradiation on RNA synthesis in mouse L cells were examined. The reduction of total RNA synthesis, the maintenance of a constant rate of RNA chain initiation and the progressive truncation of 45 S ribosomal precursor RNA, all as a function of increasing u.v. dose, suggest that premature termination of transcription is the major effect of u.v. irradiation on transcription of the L cell genome. The decrease in the production of 18 S and 28 S ribosomal RNA, and 45 S ribosomal precursor RNA is exponential as a function of u.v. dose; the respective D37 values are 310 erg/mm2, 130 erg/mm2 and 90 erg/mm2. From these inactivation kinetics, we conclude that (1) 18 S rRNA is proximal and 28 S rRNA is distal to the promotor terminus of every 45 S precursor RNA, (2) each cistron coding for 45 S precursor RNA is identical and has its own promotor locus, and (3) partial 45 S RNA transcripts that contain one or both of the mature rRNAs are processed at normal rates. In addition, for at least 90 minutes after ultraviolet irradiation, there is no observable repair of photoproducts responsible for premature termination of transcription.