Comparative investigations were made with two groups of cows (a control) and an experimental one) aiming to establish the effect of prolonged day length during the autumn-winter and the early spring periods on the physiological state of the cows, the yield of milk and fertilization during the lactation period. Results obtained indicate that supplementary illumination with visible rays at a rate of 5 w/m2 from the floor and a 16 h day length including the natural day period, enhances the content of Ca, inorganic P and vitamin D and A of the blood serum, stabilizes at a constant level the quantity of total protein and rises the content of albumins and gamma-globulins. As a result of these and probably of some other changes in the organism, the physiological state of the cows is either maintained normal or is improved, a fact leading to 13% higher milk yield on the average for the lactation period, service period shortened to 87 days and increased number of pregnant cows following the first insemination.