The study reported examined the physiological changes between two groups of 6 Friesian-cross cows in mid-lactation; one group grazed outdoors without shade, the second group was housed indoors in an open-sided barn and fed cut-and-carry forage from the grazing area. All cows were machine-milked at 05.00 and 15.30 h daily. At 14.00 h dry bulb temperatures averaged 29·5 °C outdoors and 29·0 °C indoors whilst black globe temperatures were 36·2 and 29·5 °C respectively. Relative humidity averaged 76% outdoors and 72% indoors, and the temperature-humidity indices were 79·1 and 78·3, respectively. Cows in the outdoor group had higher rectal temperatures (40·4 v. 39·0 °C; P<0·01), respiratory rates (87·9 v. 62·9 breaths/min; P<0·01) and skin temperatures (41·2 v. 38·2 °C; P<0·01) than those indoors, but lower haematocrit (26·2 v. 34·9%; P<0·01) and haemoglobin levels (10·4 v. 13·2 g/100 ml; P<0·01). Cows in the outdoor group had a higher sweating rate than those indoors (559·7 v. 68·6 g/(m2×h); P<0·01), largely due to a high radiant heat load (as indicated by slightly higher black globe readings). The results of the current study show that the barn used in this study was effective in protecting cows from solar heat. Further field studies to determine the effectiveness of lower cost strategies to reduce heat stress, including showers and shade that can be accessed by grazing cows, are needed.