The spontaneously hypertensive rat (SH) has been widely utilized as an animal model for the study of physiological functions as they pertain to essential hypertension in man. In a recent report, Wright et al. (1) demonstrated a marked decrease in the responsiveness to thermal stress in the SH rat which they, in part, attributed to deficient water mobilization for evaporative cooling. The possibility remained that changes in metabolic heat production, cardiovascular adjustments to heating or other factors which might accompany the development of hypertension could influence thermoregulation of hypertensive animals in hot environments.The work of Kloetzel et al.(2) demonstrating a high percentage of hypertensive individuals in industrial jobs requiring heat exposure has emphasized the need for studies to define stress responses in diseases such as hypertension in which the early symptoms may go undetected or the use of maintenance therapy combined with peer group pressure may encourage normal work and recreational activities. The limits which are described for physical exertion and exposure to environmental factors are nearly always based on data obtained from young, healthy individuals, and it may be reasonable to question their applicability to individuals exhibiting chronic alterations in physiological functions. In the present study, oxygen consumption and rectal temperatures of SH and normotensive rats were determined during exposure to a range of environmental temperatures designed to induce mild cold or heat stress. In an attempt to determine the nature of the observed differences, the effects of beta adrenergic blockage on 0 2 uptake in cool and warm environments was examined.Materials and methods. Male SH rats (blood pressure = 163 f 7 mm Hg and weight, 341 & 7 g) of the Okamoto-Aoki strain (3) and parent strain Wistar-Kyoto (WKY) normotensive (bp = 112 f 5 mm Hg and weight, 447 f 16 g) rats were examined at 15-20 weeks of age. Animals were housed at 24 f 1" and 50% relative humidity with a 14 hr light-10-hr dark photoperiod. Water and Purina rat chow were available ad libitum.Oxygen utilization was determined using the open flow system of Ben-Porat et al. (4). The animals were studied in 3.0 liter metabolic chambers submerged in a 170 liter waterbath held at 2 0.1" of the desired temperature. Room air was passed through the chamber at approximately 0.6 liter/min and the ambient temperature (Ta) in the chamber monitored with thermometers inserted into the chamber space. Water was absorbed on dryrite which had been incorporated into the inflow and outflow lines and in the chamber floor. Carbon dioxide absorbant (NaOH) was placed in the chamber floor and outflow lines. Air samples of about 1.0 liter were collected in Saran plastic bags at 30-min intervals over a 3.0-hr period for determination of 0 2 content with the Beckman E-2 0 2 analyzer. In order to avoid variations in the data associated with the early adjustment of the animal to the chamber environment, only the last three values were averaged and com...