Circadian fluctuations in the effect of ambient temperature (Ta) on heat production (Htot) and its relation to posture were investigated in young calves in this study. Twenty-three 6-d-old Holstein-Friesian male calves were assigned to one of four Ta treatments: 5, 9, 13, or 18 degrees C. Heat production was measured per calf continuously every 9 min by indirect calorimetry for 5 d. The posture during these 9-min periods was derived from physical activity measurements by Doppler-radar meters. Heat production varied within a day; it was highest when calves were drinking (milk or water). The influence of Ta on Htot was larger for the light (including feeding periods) than for the dark phase of the day, being related to the larger Ta effect during the feeding periods. Lower critical temperatures (LCT) were 14.1, 15.2, and 16.8 degrees C and extra thermal heat productions below LCT (ETH) were 8.48, 8.28, and 11.55 kJ.kg-.75.d-1.C degrees-1 for the dark, the light (excluding feeding periods), and the feeding phase during the day, respectively. Time spent standing was not affected by Ta but varied during the day (24-h period). Averaged over Ta, 51% of the within day variation in Htot was accounted for by the calf's posture. Correction of Htot for the time spent standing reduced the difference in both ETH and LCT between phases of the day. The present study demonstrates that circadian fluctuations exist in the thermal requirements of young calves. Part of these fluctuations are related to within-day variation in time spent standing.