Metabolic, respiratory and haematological parameters were investigated for the Little Pocket mouse during circadian torpor cycles. The rate of O2, consumption decreased from 7.04 to 0.05 ml O2· g-1· hr-1, with a corresponding decrease in respiratory minute volume from 49.4 to 0.9 ml · min-1 during torpor at an ambient temperature of 10 C. No changes in haemoglobin concentration (19.7 g/100 ml), haematocrit (54 %), red blood corpuscle count (12.4 106/μl), mean corpuscular volume (43.6 μm3), mean corpuscular haemoglobin content (16.2 pg), mean corpuscular haemoglobin concentration (37.4 %) and [2,3-DPG] (9.6 μmol/g Hb) were observed during torpor cycles. The half saturation tension of P. longimembris haemoglobin was 41 mm Hg (37 C, pH = 7.28) and 19.7 mmHg (10 C, pH = 7.51).The effect of temperature on P50was Δlog P50/ C = + 0.0106(pH = 7.4). Venous blood parameters were: euthermic mice (37 C); PCO2 = 36.8 mm Hg, PO2 = 49.5 mm Hg, pH = 7.28,[HCO3-] = 17.3mmol/l ; torpid mice (10 C) ; PCO2 = 14.6,PO2 = 35.7,pH = 7.51,[HCC3] = 18.8. These data indicate a new, relatively acidotic acid-base status during torpor, characterised by a higher H+/OH- ratio. The respiratory sensitivity to inspired CO2 of pocket mice was, despite their being semi-fossorial, typical of other mammals. High concentrations of CO2, did not induce, or facilitate, entry into torpor. © 1977.