Animal thermoregulation may have significant costs and compete directly or indirectly with other energetically demanding processes, such as immune function. Although the subterranean environment is characterized by thermally stable conditions, small changes in ambient temperature could be critical in shaping immunity. However, little is known about the effects of ambient temperature, in naturally varying ranges, on immunity of wild species. Therefore, to evaluate the effect of short-term exposure to ambient temperatures on energy metabolism and body temperature during the acute phase immune response (APR) in the subterranean rodent Ctenomys talarum, 70 adult animals were divided into three experimental groups and exposed twice for 1 h to 15, 25 or 32°C (below, at or near the upper limit of the thermoneutral zone, respectively) before and after injection with saline (control) or lipopolysaccharide (LPS, which induces the APR). Animals exposed to 25 and 32°C showed a similar APR pattern, characterized by fever (average: 37.1 and 37.7°C, respectively), a 16% increase in O2 consumption and an increase in the neutrophil/lymphocyte ratio (N/L). Body mass loss and symptoms of sickness behavior were detected from 3 and 1 h post-injection, respectively. Individuals exposed to 15°C increased their metabolic rate by 60%, showed frequent hypothermia (34.3°C on average) and the characteristic N/L increase was attenuated. Body mass loss and sickness behavior were mostly detected 24 h post-injection. Our results suggest that the thermoregulation costs in C. talarum may limit the energy available for immunity, leading to different strategies to cope with infection.