In the nucleus of higher eukaryotes, maturation of mRNA precursors involves an orderly sequence of transcription-coupled interdependent steps. Transcription is well known to influence splicing, but how splicing may affect transcription remains unclear. Here we show that a splicing mutation that prevents recruitment of spliceosomal snRNPs to nascent transcripts causes co-transcriptional retention of unprocessed RNAs that remain associated with polymerases stalled predominantly at the 3' end of the gene. In contrast, treatment with spliceostatin A, which allows early spliceosome formation but destabilizes subsequent assembly of the catalytic complex, abolishes 3' end pausing of polymerases and induces leakage of unspliced transcripts to the nucleoplasm. Taken together, the data suggest that recruitment of splicing factors and correct assembly of the spliceosome are coupled to transcription termination, and this might ensure a proofreading mechanism that slows down release of unprocessed transcripts from the transcription site.