During the day mares have different and varying mass proportion of protein and fat. The variability of fat content in the milk of mares during the day reaches 60%. In order to explain the nature of this phenomenon a series of experiments were conducted, including milking mares continuously throughout the day. The authors explain the differences in the variability of fat and protein in the milk of mares during the day by various ways of secretion from secretory cells of the udder of the components of milk. The protein secreted using merokrine type of secretion, and secretion of fat is carried out mainly by apocrine type. Due to the significant increase of milk yield in the early morning, secretion of synthesized fat slows down, which causes a reduction of fat content in the milk of mares. Secretion of accumulated fat increases with decreasing milk yield, increasing the fat content in milk. It is found that milk yield of mares on day 16,39 % higher than at night. It is also established that during the day the mass fraction of protein 5,24 % higher than the mass fraction of protein at night. The fat content in the milk of mares during the day to 12,24 % higher than the fat content in the milk of these mares at night. The correlation between daily milk yield and fat content amounted to 0,22, between daily milk yield and mass fraction of protein +0,44, between a mass fraction of fat and mass fraction of protein +0,47.