Cellular thermogenesis in L-929 mouse fibroblasts growing in glass ampoules was monitored over 72 h in three different systems : (1) closed ampoules with 2.5 mi of control medium; (2) closed ampoules with 2.85 mi of medium and a reduced oxygen content; and (3) ampoules kept open. Thermogenesis was determined as heat production (HP) and the pH, and oxygen, glucose and lactate contents in the medium as well as the cellular growth were assessed in order to evaluate whether small gaseous volumes are sufficient to facilitate adequate oxygen supply. Oxygen deficiency resulted in lower HP, decreased growth and enhanced anaerobic rate of glycolysis. Low gas volume was associated with decreased HP after 40 h and a steady increasing rate of aerobic glycolysis similar to that shown during O-2 deficiency. In the open system a more pronounced decrease of HP was observed.