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A coordinated field and chamber experiment assessed breed, sire, and cow differences in body temperatures and respiratory rates. During each of 4 yr, 10 lactating Ayrshire, Guernsey, Holstein, and Jersey cows, 40 animals per yr, were observed in the field on 10 d when the air temperature approached 32 degrees C. For the first 3 yr, each cow also was exposed in a controlled atmospheric chamber at 40 degrees C and 34 mm Hg vapor pressure. Significant breed differences were found for body temperature and respiratory rate. The order of tolerance to hot conditions was Jersey, Guernsey, Ayrshire, and Holstein both in the field and chamber. Significant sire differences, interpreted as genetic, were found for the field measures but not in the chamber. Repeatability of individual field observations for a cow on different days in a year ranged from .38 to .55. Correlations between the average of 10 observations for a cow in the field during a year and the chamber evaluation were positive but lower than the repeatability values. Correlations of ambient temperature with body temperatures and respiratory rates were highest; those for radiation were next, followed by vapor pressure and air movement.