Thirty growing yaks Bos grunniens or Poephagus grunniens, 1.0-3.5 years and 50-230 kg, from their native altitudes (3000-4000 m), were used to study the basal metabolism in this species and to evaluate the effects of high altitude and season on the energy metabolism. Fasting heat production (FHP) was measured at altitudes of 2260, 3250 and 4270 m on the Tibetan plateau in both the summer and the winter, after a 90 d adaptation period at each experimental site. Gas exchanges of the whole animals were determined continuously for 3 d (4-5 times per d, 10-12 min each time) after a 96 h starvation period, using closed-circuit respiratory masks. Increasing altitude at similar ambient temperature (Ta) did not affect (P>0.10) FHP in the summer, but decreased (P<0.05) it at different Ta in the winter. However, the decrease of FHP in the winter was mainly due to the decrease of Ta instead of the increase of altitude. In the summer, the respiratory rate, heart rate and body temperature were unaffected by altitude, except for a decrease (P<0.05) in body temperature at 4270 m; in the winter, they were decreased (P<0.05) by increasing altitude. In both seasons, the RER was decreased (P<0.05) by increasing altitude. At all altitudes for all groups, the daily FHP was higher (P<0.05) in the summer (Ta 6-24 degrees C) than in the winter (Ta 0 to -30 degrees C), and the Ta-corrected FHP averaged on 920 kJ/kg body weight(0.52) at Ta 8-14 degrees C and on 704 kJ/kg body weight(0.52) at Ta -15 degrees C respectively. We conclude that in the yak high altitude has no effect on the energy metabolism, whereas the cold ambient temperature has a significant depressing effect. The results confirm that the yak has an excellent adaptation to both high altitude and extremely cold environments.