Three groups of White Leghorn cocks were fed rations containing 2557, 2956 and 3353 kcal/kg of metabolizable energy over a 40-week period. The first, the control ration, provided the approximate maintenance level when daily intake did not exceed 90 g/bird. Semen was collected ten times from each male by abdominal massage during the experiment and the fertilizing capacity of each ejaculate was determined by artificial insemination. At the end of the experiment, the males on the 2956- and 3353-kcal rations weighed 12·5% and 18% more than at the start. Both the thickness of the layer of abdominal-wall fat and the weight of this layer plus the gizzard fat increased as caloric intake increased. Obesity, within the limits obtained in these experiments without artificially stimulated hyperphagia or forced-feeding, had no significant effect on testicular weights or on the volume and sperm density of semen. There were no consistent significant differences in the fertilizing capacity of the semen, as determined by artificial insemination, nor in the hatchability of fertile eggs.