The template activity of DNA for ribosomal RNA transcription has been investigated in monkey kidney CV-1 cells after angelicin photosensitization in order to monitor the induction of lesions in the DNA and their possible subsequent disappearance, i.e. repair. Separate confluent cultures were submitted to a single angelicin treatment at different time intervals before incubation with [3H]uridine. The labeled RNA prepared from whole cells was analysed by polyacrylamide-agarose gel electrophoresis. The results indicate that: Angelicin monoadditions on DNA constitute transcription-terminating lesions which depress overall RNA synthesis, give rise to shortened RNA chains and modify the expression of transcriptional linked genes. CV-1 cells are able to repair, at least partially, the induced transcription-terminating lesions and progressively recover RNA synthesis with a reversion of the initially observed modifications. The repair seems to be independent of semiconservative DNA synthesis since fluorodeoxyuridine does not affect the recovery of RNA transcription. The present work also confirms the arrangement of rRNA genes in tandem behind a common operator in the order 18--28 S as previously determined in the same cells by a radiological mapping technique and reinforces the potential applicability of transcription analysis to the study of repair processes operating on physically or chemically induced damage in DNA.