The maturation of mRNAs in Trypanosoma brucei involves a novel step, in which a short capped sequence is spliced in trans onto the 5' end of nascent mRNAs from a 140-nucleotide precursor. This precursor is called the mini-exon-derived RNA or medRNA. We have used drugs and ultraviolet irradiation as inhibitors to probe the synthesis and processing of medRNA in vivo. Inhibition of RNA synthesis by chloroquine shows that the half-life of medRNA is about 4 minutes. Despite this high turnover, only limited accumulation of medRNA could be achieved following a block in the synthesis of high molecular weight splice acceptor substrates by UV irradiation. This implies that there is a constraint on the steady-state levels of medRNA and that excess medRNA is degraded in the cell. A 3' shortened version of medRNA accumulates upon a block in normal medRNA processing by UV irradiation or upon treatment of the cells with actinomycin D or novobiocin but was shown not to participate in trans splicing, making it a likely candidate for an in vivo degradation intermediate.