Discontinuous mRNA synthesis in trypanosomes is thought to involve a 140-nucleotide precursor, called the mini-exon-derived RNA or medRNA, which contributes its 5' 35 nucleotides to the 5' end of nascent mRNAs. We used in vivo labelling of RNA to show that medRNA has a half-life of less than 6 min, whereas putative high mol. wt intermediates containing the 3' part of the medRNA have an average half-life of less than 1 min. This eliminates priming of pre-mRNA synthesis by intact medRNA as the main mode of discontinuous mRNA synthesis. Potential intermediates of 35 and 105 nucleotides were labelled in parallel with medRNA, but their significance could not be assessed in RNA preparations containing medRNA, as they are also produced by artefactual cleavage of medRNA. We show, however, that high mol. wt RNA, free of medRNA, can release medRNA segments upon a debranching treatment. These results are consistent with a trans splicing mechanism involving short-lived forked intermediates, analogous to lariats in cis splicing systems.