Sulphur hexafluoride (SF₆) released from permeation tubes is used as a tracer gas for the estimation of ruminal methane emissions and (e.g . Ulyatt et al. 2002) there is usually a wait of seven days or longer after tube insertion to allow intra-rumina] SF₆ to plateau prior to sampling. Our theoretical calculations, however. suggested that intra-rumina] SF₆ would plateau within 2-3 h of permeation tube placement. Because permeation tubes being developed (Hegarty and Woodgate 2003) have release rates ~ I00 times higher than those used in previous studies they run for shorter periods, and the time available for calibration and achieving plateau is critical. Permeation tubes releasing an average of 170 mg SF₆/d (Hegarty and Woodgate 2003) were placed via rumen cannulae into two cows . Rumen gas was regularly sampled (900 ml) through the rumen cannulae over a 5 d period, collected in Tedlar gas sampling bags and immediately analysed for CH₄ and SF₆ concentrations.