A speculative model is presented that proposes specific mechanisms for effecting co-transcriptional splice site selection in pre-mRNAs. The model envisions that certain splicing factors containing arginine-rich, positively charged regions bind via these positive patches to the hyperphosphorylated, negatively charged tail of elongating RNA polymerase II. Thus tethered to the transcription machinery, these splicing factors gain access to signals in nascent transcripts as they emerge from the polymerase.