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Heat shock transcription factors (HSFs) function to regulate the expression of heat shock proteins (hsps) or molecular chaperones in the cell. Mammalian cells have two well-characterized HSFs, HSF1 and HSF2. HSF1 functions to regulate the stress-induced expression of hsps. The function of HSF2 appears to be in regulating hsp expression during development and differentiation. In this work, I describe two distinct HSF1 mRNA isoforms (HSF1-a and HSF1-b) that are generated by alternative splicing of the HSF1 pre-mRNA. The two HSF1 mRNA isoforms result from the inclusion (HSF1-a), or omission (HSF1-b), of a 66 nucleotide exon of the HSF1 gene, which encodes a 22 amino acid sequence. These results show that the levels of the HSF1-a and HSF1-b mRNA isoforms are regulated in a tissue-dependent manner, with testis expressing predominantly the HSF1-b isoform while heart and brain express primarily the HSF1-a isoform. In addition, I describe two distinct HSF2 mRNA isoforms (HSF2-a and HSF2-b) that are generated by alternative splicing of the HSF2 pre-mRNA. The two HSF2 mRNA isoforms result from the inclusion (HSF2-a), or omission (HSF2-b), of a 54 nucleotid