Genetically hypertensive animals such as spontaneously hypertensive rats (SHR) and mice are more sensitive to thermal stress than normotensive controls. Genetic breeding experiments have demonstrated that the gene responsible for thermosensitivity segregates with an increase in blood pressure in the F2 generation and represents a genetic locus of hypertension. Due to a higher transcription rate of the heat shock protein 70 (hsp70) gene, which is a major heat stress gene, the accumulation of hsp messenger (m)RNA is increased in hypertension. Higher thermosensitivity and increased hsp mRNA accumulation are also observed in neonatal cardiomyocytes and cultured vascular smooth muscle cells from SHR, suggesting that these abnormalities are primary in character. This higher hsp70 transcription rate in hypertension could be due to an abnormality in the promoter region, to an interaction between heat stress trans-acting factor and heat stress element within the promoter of hsp70 or to an abnormal activation of heat stress trans-acting factor. A study using recombinant inbred animals has indicated that RT1 complex gene(s), a major histocompatibility complex in the rat, may be involved in the development of hypertension. These findings, together with the fact that hsp70 is located in the major histocompatibility complex, suggest that hsp70 gene or associated genes within the RT1 complex are responsible for environmental control of the expression of hypertension.