The records of 2400 cows on test at the London Dairy Show between the years 1922 and 1934 have been examined for the relationship between the gross physiological efficiency of milk production and the factors of breed, size of cow within a breed, actual yield of milk and stage of lactation. (“Gross Efficiency” is here the ratio of energy in the milk to the energy in the digestible nutrients consumed.)(1) It is shown that among the best representatives of the various dairy breeds there is little difference in gross efficiency of milk production. The lactation stimulus has been bred approximately in proportion to the size of the breed.(2) Cows, in spite of their greater weight, are more efficient than heifers, and animals milked thrice daily than those milked twice daily.(3) Within the breed there is a slight though definite tendency for gross efficiency to decrease with increase in live weight.(4) Greater than any of the foregoing differences in gross efficiency is that which exists within a breed due to differences in actual milk yield.(5) Stage of lactation has also much to do with gross efficiency. There is a steady decline in efficiency with advance in lactation from 38·75 to 29·25 per cent.