Abstract: The genome-wide expression pattern of budding yeast Saccharomyces cerevisiae in response to mild heat treatment in a non-fermentable carbon source was analyzed using DNA microarrays. Of 5,870 open reading frames (nuclear genome transcripts) examined, 104 genes were upregulated and 287 genes were downregulated upon shifting of the cells from 25 degrees C to 37 degrees C. Forty upregulated genes and 235 downregulated genes encoded localization-assigned proteins. Of 113 heat-repressible genes (excluding 122 heat-repressible ribosomal genes), 36 were mitochondria-related genes, whereas only 2 of 40 heat-inducible genes were mitochondria-related. In particular, 9 genes involved in the mitochondrial respiratory chain and 7 genes involved in mitochondrial protein translocation were significantly repressed, suggesting that mitochondrial respiratory function and biogenesis were downregulated. Consistent with these findings, the growth of yeast cells in a non-fermentable carbon source was repressed at 37 degrees C and the mitochondria isolated from heat-stressed cells exhibited compromised preprotein-import activity compared with those from unstressed cells. In contrast, many genes involved in glycolysis and the metabolic pathway to produce glutamate via the tricarboxylic acid cycle, which is essential for biosynthetic reactions, were upregulated. Yeast cells might downregulate mitochondrial function to circumvent heat-induced oxidative stress, upregulate stress-related genes, and remodel genes for metabolic pathways in response to mild heat stress: an adaptive response at the expense of cell growth.