Cotranscriptional loading of proteins onto nascent transcripts contributes to the formation of messenger ribonucleoprotein particles (mRNPs) competent for nuclear export. The transcription machinery is believed to play a pivotal role in mRNP assembly, which is at least partially linked to the function of the THO/TREX complex and the mRNA termination/polyadenylation apparatus. Here we demonstrate a prominent role for the rate of transcription in the production of export-competent mRNPs. We show that a transcription-defective allele of the Rad3p helicase, a component of the TFIIH transcription initiation factor, suppresses several phenotypes associated with defective mRNA processing and export. Strikingly, the effects of compromised Rad3p activity can be phenocopied by a transcription elongation drug as well as by other mutations affecting transcription. Our results suggest that efficient mRNP assembly is under a kinetic control that is influenced by the rate of transcription.