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The study included 37 cows, 20 of which were of the Holstein-Friesian and 17 of which were of the Brown Swiss breed at the Agricultural Institute's farm in Stara Zagora. The cows from both breeds were kept together under similar rearing and feeding conditions. A free-range production system with individual boxes for rest was applied. The cows were divided into three technological groups depending on their physiological condition, respectively: dry period, first lactation period (up to the 120th day of lactation) and second lactation period. Feeding was based on a complete ration including maize silage, alfalfa haylage, concentrated feed, and vitamin mineral premix. The concentrated feed during lactation was in accordance with the group's mean milk yield. The cows' body condition score (BCS) was measured monthly per a 5-grade evaluation system with accuracy of up to 0.5 points. The body condition score of the cows at calving was measured 7 to 10 days before calving. The BCS at calving had a significant influence on the 305-day lactation milk yield, test-day milk yield and peak lactation production. The cows that reached a BCS of 3.5 - 4 points at calving had sufficient body reserves for the lactation's beginning, which allowed them to reach higher milk yield during the peak of lactation, and higher yield for 305 days compared to cows with grades of 3 or lower at calving. The Holstein-Friesian cows had better mobilisation potential than Brown Swiss cows. At BCS of 2 to 2.5 points at calving, Holstein-Friesian cows reached a milk yield that was only 876 kg less than those of cows with high BCS (3.5 - 4 points), whereas in Brown Swiss cows the difference was 1,400 kg. The cows of the Brown Swiss breed had preserved to a greater extent the defence reaction to reduce milk production when body reserves were diminished.