Previous experiments have suggested that estimates of methane (CH 4) emissions from ruminant animals made using the sulphur hexafluoride (SF6) tracer might be influenced by the permeation rate of SF6 (PR). This study examined the latter issue with cattle. For this, analyses of data sets from two grazing trials involving large herds (exps. 1 and 2) and a specifically designed controlled trial (exp. 3) were conducted. Individual daily CH4 emissions from 296 (exp. 1) and 388 (exp. 2) Friesian x Jersey cows in mid-lactation were measured with herds subdivided into four (exp. 1) or five (exp. 2) measurement groups and dry matter intake (DMI) estimated by energy metabolism algorithms. The ranges of tracer PR in exps. 1 and 2 were 2.624-5.689 and 2.214-3.594 mg d-1, respectively. Experiment 3 was conducted using 12 rumenfistulated beef steers pen-fed on lucerne silage and design arranged as a 4 x 4 Latin square with three replications. Permeation tubes with four levels of nominal PR (three tubes each): low (L), medium (M), medium-high (MH) and high (H) were randomly assigned to four rumen deployment sequences (L-M-MH-H, H-MH-M-L, MH-L-H-M and M-H-L-MH). The grazing experiments revealed a positive effect of PR on the CH4 emission estimates (1 mg SF6 d-1 counting for 0.6-2.3 g kg -1 DMI), but this effect was significant (R2 =0.06-0.23, P <0.05) only when there was a large range in PR (exp. 1), whereas with a narrower PR range (exp. 2) the effect was not significant (R2 <0.04, P>0.05). Experiment 3 revealed that the influence of PR upon CH4 emission estimates was linear. It is concluded that despite an influence of PR on CH4 emission estimates, accuracy and precision of the tracer technique is warranted provided that PR are used in a narrow range and balanced between the experimental treatments.