Summary Using 18 bulls, 9 Holstein-Friesians and 9 Guernseys, studies were conducted relating measurable semen characteristics, ‘relative fertility, and body weight changes to T.D.N. intake levels of approximately 100, 120, and 140 per cent of recommended maintenance requirements for dry dairy cows of equivalent weights fed simultaneously with concentrate mixtures containing 12,16, and 20 per cent total protein. These relatively wide ranges of T.D.N. intake, accompanied by considerable shifts in body weight, can be fed mature bulls for at least 90 days without influencing semen production or fertility. Under the conditions of this experiment, 12 per cent of protein in the concentrate mixture fed with mixed hay, containing approximately 10 per cent of legumes, supplied enough protein for semen production and the maintenance of fertility. Based on the results of this experiment, a tentative feeding standard for mature dairy bulls used routinely in artificial insemination is proposed. In terms of a practical feeding schedule, such a standard reduces itself to approximately 1 lb. of hay and 0.4 to 0.5 lb. concentrate mixture daily per 100 lb. of body weight. The authors are indebted to R. F. Elliott, Laboratory of Animal Nutrition, Department of Animal Husbandry, Cornell University, for the chemical analysis of the feeds used.