The analysis concerns the size of the ventilation air stream, which is variable in time, in a cowshed with natural ventilation. The research was carried out in two buildings designed for dairy cattle, one has ventilation ducts, the other an air vent. The research covered measurement of internal and external air temperature, which is variable in time, air flow temperatures and velocity in air vents and the wind velocity. Temperature and velocity of air flow in particular ducts differed between each other. In the winter season, average air temperature in ducts was lower than the air temperature in the zone where animals stayed. The ventilation air stream size resulting from the measured flow velocities in ventilation ducts was compared to the theoretical size.