Abstract. The research has been carried out with cows of the Bulgarian black-and-white cattle in 10 farms from various regions in the country. The cows calved in the period 1995 – 2006. A total of 1460 cows are included with 2852 lactations from І-st to ІІІ-rd and respectively 27228 test day records. The age at first calving has reliable effect on test day milk yield, milk fat and % of fat. Cows that calved for the first time at an age of 27 to 30 months have the highest test day milk yield and milk fat followed by the ones that calved respectively on 24-27 and 30-33 months, with small difference between them. The cows that calved at the age of 27-30 months had the highest daily milk yield in the lactation peak – 19.78 kg. In the test day % of fat the tendency is for higher values in cows that calved for the first time at a greater age.