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Reproductive performance of 12 11-month-old Yorkshire boars was determined during and after exposure to control or increased ambient temperatures. After 3 weeks adjustment at 23 C in temperature controlled chambers, six boars were heat stressed by exposure to 34.5 ± 1.0 C for 8 hr and 31.0 ± 1.0 C for 16 hr during each 24-hr period and six control boars were maintained at 23.0 ± 1.0 C for 11 weeks. All boars were subsequently exposed to 23 C for 6 weeks. Semen was collected twice weekly before, during and after heat stress and boars were naturally mated with gilts during weeks 7 to 11 of treatment. Daily rectal temperatures and respiratory rates were increased during heat stress, but the ability to ejaculate and semen volume were not altered. Percentage motile sperm was decreased during heat stress (P<.005) and did not return to normal values until 5 weeks after the end of exposure to increased ambient temperatures. Daily sperm output from heat stressed boars was decreased (P<.025) during daily ejaculation on days 33 to 38 of treatment and averaged 54% of the total sperm ejaculated by control boars. At 30 days after natural breeding only 59% (P<.05) of the gilts bred by heat stressed boars were pregnant compared to 82% for gilts mated to control boars.