Heat stress triggers physiological and metabolic adaptations in lactating dairy cows reducing gut absorption and net portal flow of amino acids. Heat stress also impairs mammary blood flow, a local mechanism affecting milk yield, and milk protein synthesis. Limited flow of amino acids to the mammary gland partially accounts for changes in milk protein synthesis. Mounting evidence indicates that mammary clearance of amino acids increases to sustain milk protein synthesis as a local mechanism to adapt to reductions in plasma amino acid concentrations. Heat-stressed cows also elicit metabolic adaptations to support milk protein synthesis when metabolizable protein supply fails to meet requirements. Recent studies also demonstrate that milk protein synthesis in heat-stressed cows is maintained despite noticeable reductions in dietary rumen degradable and undegradable protein contents. Our research indicated that a 690 g reduction of metabolizable protein supply led to a 60 g reduction in milk protein yield; however, energy-corrected milk yield was not affected. Plasma analysis revealed that insulin concentrations declined by 45% and nonesterified fatty acids increased by 40% in association with lowering metabolizable protein supply in heat-stressed cows. Insulin responds to physiological state and nutritional status, and inhibits mobilization of fatty acids reducing fuel availability for oxidation and precursors for milk fat synthesis. Thus, reduction of insulin may partly mediate improvements in the utilization of amino acids and nonesterified fatty acids and provide substrate for milk synthesis in the heat-stressed cow. The insulin-signaling pathway may also be affected by changes in circulating insulin during heat-stress. The signaling pathway mammalian target of rapamycin is a promising candidate to study because it integrates physiological and nutritional signals to affect protein synthesis and nutrient utilization in the mammary cell. Future studies testing nutritional interventions on physiological adaptation to heat stress will continue to improve our understanding and will provide feeding strategies to improve cow productivity.