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Recent studies suggest the importance of hair coat in cattle adaptation to heat stress environments. The objective of this study was to quantify the physiological responses of Holstein cows to two different loads of solar radiation (lo-550W/m2 and hi-880W/m2). These solar loads represent the am (1000-1200) and pm (1300-1600) periods in Arizona summers. Cows (dry and pregnant) of two hair coats: a) black (n=4, >90% black) and b) white (n=3, >90% white; n=1, 60% white) were used in a switched back design in an environmental chamber study with or without solar radiation. Each exposure to solar radiation consisted of 3 days with 2 white and 2 black cows. Daytime THI for the chambers were set at 84 (38.9°C, 25% RH) and nite time THI were set at 70 (23.3°C, 56% RH). Physiological responses measured were: a) respiration rates, b) dorsal skin temperations, c) rectal temperatures and d) sweating rates. Black hair coat absorbed 91% of the solar load while white hair coat absorbed 57%. Dorsal temperatures for black or white coat at lo solar were 39.5°C and 34.5°C and at hi solar were 41.7°C and 34.9°C, respectively. Respiration rates were not different for the coat colors; lo solar 86 breaths/min. and hi solar 99 breaths/min. Rectal temperatures at lo solar were 38.9°C for black and 38.7°C for white and at hi solar were 39.4°C for black and 39.0°C for white. Cows with black hair coat had 1.5x higher sweating rates than cows with white hair coat at both solar loads. Variations in the ability to thermal regulate by sweating within a hair coat color were observed. The study suggested that cows with black coat absorbed higher solar load and this was compensated by higher sweating rates facilitated by higher dorsal skin temperatures thus allowing for greater evaporative cooling. [ABSTRACT FROM AUTHOR]