EN
The effect of inbreeding on milk and fat yield and fat content in milk in the first three lactation periods of red-white cows was determined by means of linear and parabolic regression. The calculations were made within the group of sires for low, medium and high production levels and irrespective of the herd's productivity. Inbreeding led to a slight depression in milk and fat yield -7.5 kg and -0.3 kg per 1% of Inbred, respectively - but it dit not affect fat content in milk. The most significant depression in milk and fat yield was observed at the level of medium productivity. It ranged from 11.5 to 17.7 kg of milk and from 0.31 to 0.51 kg of fat according to the lactation, with the Increase in the inbred by 1%. The highest inbreeding lactation; depression in milk and fat yield however remained rectilinear.