Exposure of rats to an environment of +40 degrees C that increases central (rectal) temperature to about 40 degrees C, importantly decrease the number of thrombocytes, the blood coagulation (the Quick time and Howel time) and euglobulin lysis time (ELT). In order to search for the mechanism responsible for coagulability and fibrinolysis we studied the relations of these changes with the sympatheticoadrenergic system and with the adrenal hormonal secretion. Some parameters were thus explored after adrenalectomy and under the influence of a beta-blocking agent, Eraldin. Adrenalectomy moderately reduced these decreasing effects, excepting the ELT. Eraldin has a contrary effect. Adrenalectomy acts by suppressing the corticoid reaction and by diminishing the catecholamineones. Since Eraldin influences coagulation and fibrinolysis in an opposite sense than adrenalectomy, it may be concluded that catecholamines do moderate the effect of heating, inhibiting and not stimulating circulation, as assumed. On the contrary, corticoids (gluco- or mineralo-) contribute to the effect, influencing coagulation in hyperthermia. Both influences are slight, not determinant. Supplementary explorations revealed that repeated daily exposure for 1-2 or 3 weeks, simply in the cages in which hyperthermia was subsequently produced, decreased also the number of thrombocites, the QT and the ELT, especially the last one. The HT was less influenced. It results that these exposures, produced effects similar with hyperthermia. Heating rats after these daily repeated exposures does not influence these already changed parameters.