The preoptic-anterior hypothalamus (POAH) controls body temperature, and thermoregulatory responses are impaired during hypercapnia. If increased CO₂ or its accompanying acidosis inhibit warm sensitive POAH neurons, this could provide an explanation for thermoregulatory impairment during hypercapnia. To test this possibility, extracellular electrophysiological recordings determined the effects of CO₂ and pH on the firing rates of both temperature sensitive and insensitive neurons in hypothalamic tissue slices from 89 male, Sprague-Dawley rats. Firing rate activity was recorded in 121 hypothalamic neurons before, during and after changing the CO₂ concentration aerating the tissue slice chamber or changing the pH of the solution bathing the tissue slices. Increasing the aeration CO₂ concentration from 5% (control) to 10% (hypercapnic) had no effect on most (i.e., 69%) POAH temperature insensitive neurons; however, this hypercapnia inhibited the majority (i.e., 59%) of warm sensitive neurons. CO₂ affected similar proportions of (Non-POAH) neurons in other hypothalamic regions. These CO₂ effects appear to be due to changes in pH since the CO₂-affected neurons responded similarly to isocapnic acidosis (i.e., normal CO₂ and decreased pH) but were not responsive to isohydric hypercapnia (i.e., increased CO₂ and normal pH). These findings may offer a neural explanation for some heat-related illnesses (e.g., exertional heat stroke) where impaired heat loss is associated with acidosis.