Fossorial mammals are frequently exposed to hypercapnic and/or hypoxic conditions in their burrows. This research compares ventilatory responses of golden hamsters (<i>Mesocricetus auratus</i>) to different ambient respiratory gas concentrations with those of the white rat (<i>Rattus norvegicus</i>). Total body plethysmography was used to measure tidal volume (V_T), respiratory frequency (R_f), and minute volume (V_E). The respiratory gases had carbon dioxide concentrations ranging from 0 to 790 and oxygen content ranging from 13 to 21%. Both hypercapnic and hypoxic gas mixtures caused hyperventilation in hamsters. The more a gas mixture deviated from normal air, the greater the ventilatory increase. Combining hypercapnic and hypoxic conditions did not potentiate the response. Rats exhibited a greater hyperventilation in response to hypercapnic-hypoxic and hypercapnic gas mixtures than did the hamsters. Hypoxia alone caused a greater response in the hamsters. Greater blood buffering capacity of hamsters offers a possible explanation for the different ventilatory responses observed between these two rodents. / M.S.