Changes in thermal environment can invoke a stress response in pigs, which in turn can potentially impact their immune system and disease susceptibility. We investigated effects of heat stress and social rank on behavior, immune responsiveness, and performance of pigs challenged with porcine reproductive and respiratory syndrome (PRRS) virus. Sixty-four 7-week-old PRRS-naïve pigs were assigned to each of four experimental treatments consisting of a 2 x 2 factorial design: PRRS (PRRS- or PRRS+) and environmental temperature (24 degrees C or 32 degrees C). Blood samples were taken prior to and at days 7 and 14 post-inoculation, and alveolar macrophages were collected via bronchoalveolar lavage at day 14. Total white blood cell counts, natural killer cytotoxicity, macrophage numbers, macrophage subpopulations, and performance measures were all significantly affected by social rank, heat stress, and/or infection status of the pig. Heat stress and PRRS status also significantly influenced the amount of time pigs spent lying with or without contacting another animal. Cortisol and various immune measures were also affected by PRRS status. These results show not only that intranasal inoculation with PRRS virus affects physiological, behavioral, and performance measures in growing pigs, but that social rank influences pigs' immune responsiveness to PRRS as well. Moreover, heat stress does not have additive negative impact on physiological or performance traits in pigs challenged with PRRS virus.