Abstract: An attempt was made at improving fertility of male broiler breeders exposed to elevated ambient temperatures. Acetylsalicylic acid (ASA, aspirin) is a potent antipyretic drug that has been shown to lower the body temperature of heat stressed chickens. Because deviation in body temperature above normal is negatively correlated with fertility, the objective of the present study was to determine if ASA would lower rectal temperature of heat stressed male broiler breeders and improve fertility. Thirty six Arbor Acres roosters were divided equally among three controlled temperature rooms and caged individually. Half of the males in each room were fed .15% ASA, while the other half of the birds received the control (C) diet. Males were fed the C and ASA diets beginning 1 wk prior to heat stress treatment. After this pretreatment period, the temperature in all three controlled temperature rooms was increased to 29oC. Following one wk at 29oC, room temperature was increased further to 32oC for 3 wk. Once every wk of the experiment, 120 hens were inseminated with 50 million sperm from either ASA or C fed males. Dietary ASA did not lower the body temperature of the heat stressed roosters. Males fed ASA consumed less feed than males fed the C diet. In general, semen characteristics, such as semen volume, sperm concentration, and percentage of dead sperm produced, were unaffected by dietary treatment. However, addition of ASA to the heat stressed male`s diet resulted in a linear decrease in fertility over Weeks 2 through 4 of the experiment and a greater reduction in fertility over days postinsemination than that obtained for males receiving the C diet. In vivo sperm-egg penetration was similar whether hens were inseminated with semen from C or ASA fed males. In conclusion, .15% ASA in the male`s diet does not decrease body temperature when roosters are exposed to elevated ambient temperatures. In addition, 0.15% dietary ASA appears to be detrimental to fertility of heat stressed broiler breeder males.