The effect of a shade structure on some physiological parameters of lactating Dutch-type Friesian cows was determined over three consecutive summer periods. Plasma cortisol concentration of no-shade cows was significantly (P < 0.05) higher than that of shade cows during the 1985/86 experimental period as well as the combined analysis over the two experimental periods. Cortisol levels were increased (P < 0.05) owing to higher temperatures during the 1984/85 period only. Plasma thyroxine concentration of the no-shade cows tended to be lower than that of shade cows. Thyroxine levels were reduced (P < 0.01) by increasing maximum ambient temperatures during the 1984/85 experimental period only, while a negative trend was observed during 1985/86. The average respiration rate and the average rectal temperature of no-shade cows were higher (P < 0.01) than those of shade cows at 11:00, 13:00 and 15:00 on hot days (maximum day-time temperature, 25.1 °C) but there was no difference at any time of the day on cool days (maximum ambient temperature -< 25.0 °C). Both respiration rate and rectal temperature of shade and no-shade cows increased (P < 0.01) with increasing ambient temperatures.