The synthesis of most, if not all, mRNA in trypanosomes involves the splicing in trans of two RNA molecules. One of these includes the common 5' mini-exon or spliced-leader sequence, while the other contains the complete protein-coding exon sequence for a given gene. We have recently shown that trans-splicing of tubulin transcripts is disrupted by heat shock, whereas trans-splicing of Hsp70 mRNA continues unabated, despite the increased expression of this latter gene [Muhich and Boothroyd, Mol. Cell Biol. 8 (1988) 3837-3846]. In this report we show that, in addition to being polyadenylated, the dimeric beta-alpha tubulin transcripts which accumulate as a result of heat shock have a correctly processed 5'-end (through mini-exon addition), and that the extent of disruption is proportional to the degree of heat shock. We further show that mRNA levels for two evolutionarily-conserved heat-shock proteins, Hsp70 and Hsp85, are increased under the splicing-disruptive conditions of heat shock.