The objective of this study conducted in tropical Brazil was to characterize some physiological responses to heat stress in imported Bos taurus, native Bos taurus, and native Bos indicus cattle. Imported Simmental (n = 107) native Simmental (n = 99), and native Bos indicus cattle (n = 121) (42 to 80 mo of age) were evaluated. Animals were walked 7 km at 37 degrees C and 60 to 65% relative humidity during midday. Rectal temperatures and respiration rates were taken before and after the walk. A .01-cm2 sample of cutaneous tissue from the lateral cervical region was obtained from each animal. Slices were stained with hematoxylin-eosin solution, and the epithelial strata were counted. Perimeter of the sweat glands was also calculated. Rectal temperatures before the walk were greater (P < .001) in imported Simmental (40.52 +/- .04 degrees C) than in native Simmental (38.92 +/- .04 degrees C) or Bos indicus (38.90 +/- .04 degrees C). Rectal temperatures after the walk were greater (P < .001) in native Simmental (39.87 +/- .05 degrees C) than in Bos indicus (39.46 +/- .05 degrees C). Because of the heat, imported Simmental were not capable of finishing the drive, and rectal temperatures could not be taken. Respiration rates before and after the walk were greater (P < .001) in imported Simmental (64.3 +/- .6; 95.8 +/- .8) than in native Simmental (35.0 +/- .6; 56.8 +/- .8) or Bos indicus (15.0 +/- .2; 33.2 +/- .8). Sweat gland perimeter was greater (P < .001) in Bos indicus (540.5 +/- 19.1 mm) than in native Simmental (382.0 +/- 27.6 micrograms) or imported Simmental 497.2 +/- 17.4 micrograms). Native Bos indicus were environmentally adapted, native Simmental had elevated body temperatures and respiration rates, and imported Simmental had dramatically increased body temperatures and respiration rates. Native Bos indicus cattle were environmentally adapted and differed in skin histology, sweat gland histometry, and number of epithelial strata.