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Four grade Jersey calves were exposed for 7 hr. twice a week to each of several atmospheres having dry-bulb temperatures ranging from 85 to 110° F. and absolute humidities ranging from 6 to 16 gr./cu.ft. The following reactions were studied: rectal temperature; pulse rate; respiratory rate; respiratory volume; evaporative loss; calcium, phosphate, sugar and erythrocyte composition of the blood; behaviour.Rectal temperatures rose rapidly to a higher level than was shown by cows under similar conditions, but maintained a steady equilibrium thereafter, except under the most severe conditions.Respiratory rate responses resembled those of rectal temperature, the difference from those of cows being even more striking.Humidity had relatively less effect upon the rectal temperature and respiratory rate responses of calves than of cows.Pulse rate and tidal respiratory volumes were relatively unaffected, but minute respiratory volumes rose.Evaporative loss per unit body weight resembled that of cows, except that humidity again had less effect. Pulmonary ventilation was inadequate to account for the observed losses.Behavioural changes resembled those of the cows but weakness of the hind limbs was observed at rectal temperatures above 106° F. Blood calcium, phosphate, sugar and erythrocyte levels were not essentially affected.Possible explanations for the lower thermal tolerance of calves, as compared with cows, are considered.

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