The objective of this study was to establish the physiological parameters of Saanen goats at absorptive and post absorptive statuses, to determine the required period of fasting for these animals. Gas exchange was recorded by an open-circuit face-mask respirometry. Six nonlactating and non-pregnant Saanen (49.2 ± 3.2 kg) goats were used in this experiment, allocated to individual pens and fed the same total mixed ration (19,5% of crude protein, 10 MJ per kg DM of metabolizable energy, ME). Gas exchange was measured in groups of 3 animals each and each data collection period lasted 6 d. During feeding period (3 d), each group was submitted to a double Latin square (3 × 3) with 3 animals and 3 h window in the morning (7.00–9.00h; 9.00–11.00h; 11.00–13.00h) and evening (13.00–15.00h; 15.00–17.00h; 17.00–19.00h). After the 3-d feeding period, animals were subjected to fasting (no feed, only water) and gas exchange measurement was performed for 30 min periods at 12, 20, 36, 44, 60, and 68 h after fasting. To evaluate the decay of methane during fasting, methane production was fitted to a nonlinear logistic model. The critical time to reach the asymptote, or to measure fast heat production (FHP), was obtained when the upper limit of the standard error of mean of minimum methane produced and when CH₄ production became equivalent. During the feeding period, Saanen goats’ drank 3.34 ± 0.53 L/d of fresh water and consumed 848.6 ± 40.2 g/d of dry matter, the daily heat production, averaged 557.4 ± 38.7 kJ/kg^0.75 BW and their FHP was 236.0 ± 19.7 kJ/kg^0.75 BW. After fasting, methane production decreased exponentially and the critical time to measure FHP was 47 h of fasting for Saanen goats when methane production was statistically equal to zero. Taken together, our results suggest that the ideal period to measure FHP should be between 40 and 60 h of fasting for goats fed at maintenance levels. The results presented herein are relevant for future energy metabolism studies in goats.