The laser methane detector (LMD) has been increasingly used in the last decade to assess the methane (CH4) concentration in the exhaled air of ruminants. The CH4 concentration was mostly measured indoors, where animals were either retained in a feed fence or manually by a person to facilitate the LMD measurements. By contrast, the use of the LMD for measurements under outdoor conditions has been limited to very few studies. The present study applied the LMD to assess the CH4 concentration in air exhaled by four pasture-fed female Thuringian Forest goats when they were either expressing their natural grazing behaviour or when they were manually restrained at three times of the day during five consecutive days. We compared the activities, including rumination, and the CH4 concentration between restrained and free-roaming goats, and between goats, days, times of the day and goats' activities. We hypothesised that the restraint influences the goats' behaviour, particularly rumination, which could lead to a change in CH4 concentration. The overall CH4 concentration (median) was not affected by goats' restraint (6.5 ppm-m for restrained vs. 6.6 ppm-m for free-roaming). However, restraint influenced goats' rumination activity, with differences between individual goats and days. A lower rumination activity was recorded on the first and the last two days as compared to Day 2-3. Despite the greater rumination activity, the CH4 concentration was smaller on Day 2 (5.8 ppm-m) as compared to Day 5 (7.4 ppm-m). Similar observations were made with respect to behaviour and CH4 concentrations in free-roaming goats. By contrast, no differences in the proportion of time of activities were found between the times of day for restrained goats, while free-roaming goats preferred to stand idle but were less frequently lying idle in late afternoon. Still, the greatest CH4 concentration in restrained goats was obtained for the midday measurements (7.0 ppm-m), while it further increased until late afternoon for free-roaming goats (6.8 ppm-m). It is concluded that the restraint of animals during outdoor measurements can facilitate LMD measurements in grazing animals without changing the results for CH4 concentration in air exhaled by the animal. An adaptation period of one day followed by two to three measurement days with sufficient measurement periods to account for different activities is recommended to limit the impact on animals' stress level.