ABSTRACT..Heat stress reduces milk production, depresses fertility and increases the incidence of health disorders in dairy cows. Our first goal was to estimate genetic components of milk yield (MY) and somatic cell score (SCS) across lactations considering heat stress. Our second goal was to reveal genes responsible for thermotolerance. Data included 254k MY and 356k SCR test-day records of 20k Holstein cows. Multi-trait repeatability test-day models with random regressions on THI values were used to estimate variance components. The models included herd-test-date and DIM classes as fixed effects, and generic and heat tolerance additive and permanent environmental as random effects. Genetic variances for MY under-heat stress increased 3.9 and 6.5% between consecutive parities, suggesting that cows become more sensitive as they age. Heritability estimates for MY at THI 78 were between 0.17 to 0.32. Genetic correlations between general merit and heat tolerance ranged from -0.30 and -0.55, indicating production and thermotolerance are antagonistic. For SCS, heritability estimates for SCS at THI 78 were between 0.10 and 0.16. For this trait, genetic correlations between general merit and thermotolerance were always positive, ranged from +0.10 to +0.43. Whole-genome scans were performed using ssGBLUP. For MY, as expected, the region on BTA14 that harbors DGAT1 was associated with general merit in all three parities. One region on BTA15 was associated with thermotolerance across lactations; this region harbors PEX16, MAPK8IP1, and CREB3L1, genes implicated in thermogenesis and cellular response to heat stress. For SCS, regions on BTA6 and BTA29 were implicated in general udder health in all parities. These regions harbor genes, such as CXCL13, SCARB2, and FAT3, that are involved in immune response. Notably, genes DLX1 and DLX2 which downregulate cytokine signalling pathway were associated with SCS thermotolerance in all lactations. Overall, this study contributes to better understanding of the genetics underlying heat stress and point out novel opportunities for improving thermotolerance in dairy cattle. Keywords: heat stress, variance components, repeatability test-day model, ssGBLUP.