In Brazil, dairy farming is an activity of extreme economic and social importance, with this sector being the basis for the livelihood of thousands of families, being present in almost all municipalities in the country. Brazil is a country with a predominantly tropical climate and even in confinement dairy production systems (or compost barn), in which environmental control is provided, the thermal comfort of the animals might not be achieved, especially when it comes to specialized breeds in free stall (e.g. Holstein). In pasture-based production systems, heat stress tends to be more pronounced than that under confinement management and its effects can become even more striking, given that the greatest production of pasture forage biomass occurs during the rainy season, which is markedly characterized by increases in temperature and relative humidity. In a recent study conducted in partnership by Embrapa Dairy Cattle and DSM company, positive effects of supplementation with calcidiol were observed in the diets of Holstein cows in the pre-partum period, particularly on positive responses in milk production and feeding efficiency , as well as plasma glucose and 25-OH D3 concentrations. These results indicated that supplementation with calcidiol can improve the energy metabolism of animals, potentially reducing the effects of heat stress on animal performance and/or promoting faster recovery of these animals after stress. Thus, this project aimed to evaluate the effect of supplementation with calcidiol on the productive performance of lactating cows under thermal stress in two different production systems, as follows: during confinement (free stall) using Holstein cows; and at pasture, using crossbred cows. In both systems, the effects of calcidiol (25-hydroxyvitamin D3; 25 (OH) D3) and vitamin D3 (cholecalciferol) were evaluated on dry matter intake, milk production; blood metabolites and urinary mineral excretion of lactating cows under heat stress. In the experiment conducted with lactating Holstein cows in a confined system, no effect of calcidiol supplementation on milk production, dry matter intake and blood metabolites was observed as a function of heat stress. However, cows fed calcidiol had lower body surface temperatures, suggesting a more active heat loss during the period of heat stress. In the experiment conducted under grazing conditions, dietary supplementation with calcidiol did not influence the dry matter intake or the productive performance of crossbred cows. However, supplementation with calcidiol promoted a reduction in urinary excretion of Ca and P without influencing blood concentrations of Ca, P and Mg, which may indicate a positive effect on the efficiency of Ca and P utilization. The information presented in this document contributes to achieving the Sustainable Development Goals (SDGs) number 1 - Eradication of poverty: End poverty in all its forms, everywhere; 2 - Eradication of hunger: End hunger, achieve food security and improved nutrition and promote sustainable agriculture; 8 - Dignified employment and economic growth: Promoting growth sustained, inclusive and sustainable economic development, full and productive employment, and decent work for all.