Fourteen lactating, non-pregnant Holstein cows were heat synchronized for estrus and assigned randomly to pens in a freestall barn with (Fan) or without (Control) a fan to observe the effect of forced ventilation on cortisol concentrations. After a 21-day adjustment period, jugular cannulae were inserted for blood sampling before PGF2α injection. All animals were in their luteal phase at the time of injection as determined by previous observations for estrus and palpation. After PGF2α. blood samples were collected at 6-h intervals from 0 to 36h, 4-h intervals from 36 to 88h and 3 times weekly for three weeks thereafter. Blood samples were assayed for cortisol. Daily rectal temperatures and ambient data were recorded. Average daily rectal temperatures were lower (P<0.05) in the Fan (39. I°C) than in the Control (39.5 °C). Cortisol did not show a diurnal pattern and the values did not differ (P >0.05) between treatment groups.