Two lactating cows were individually exposed, in a temperature- controlled room, to diurnally varying temperatures ranging from morning ambient temperature of 21 °C to a peak afternoon temperature of 34°C for a period of 10 weeks after an initial period of 8 weeks in the ambient temperature of 21°C. In the 18 weeks period, mostly during the period of exposure to the chronic heat, the following changes in physiological reactions were observed in one of the cows, progressive reduction in hair coat, skin and rectal temperature (P<0.01). These changes in the temperature would be expected to enable the cow to maintain a continuous temperature gradient between deep body temperature and surroundings thus allowing it to lose heat via conduction, convection and radiation. Both tissue and coat insulations declined with acclimation to the chronic heat where tissue insulation declined significantly (P<0.01). These changes were also expected to allow the cow to be better able to dissipate heat in the hot environment. As the cow got acclimated to the chronic heat, its rate of sweating increased with earlier outbreak of sweating during the day. This is presumably a result of onset of sweating at lower skin temperature. This might be expected to contribute greatly to the increase in tolerance in dry heat and may also be useful in wet heat. Though both respiratory and pulse rates showed some changes during acclimation, the observed changes were not statistically significant. It might be that acclimation to chronic heat does not impose too much of a stress on the heat balance mechanism of the animal hence clinical effects, in terms of increased respiratory and pulse rates, might not be an attribute of cattle experiencing chronic heat. Production parameters, such as feed intake, water consumption, metabolic rate and milk yield showed some changes with acclimation but these observations might have as well been confounded by the declining milk yield with advancing lactation. It was therefore not possible to attribute the changes observed in these parameters sorely to the heat treatment. Probably, if the effects of declining milk yield with advancing lactation could be accounted for, the effects of chronic heat on these production parameters might become apparent.