In vitro gas production techniques are becoming increasingly popular as tools to investigate the food quality for ruminant and monogastric herbivores. They are attractive due to their ability to measure the kinetics of digestion associated with the gut microbial fermentation of foodstuffs. It is therefore important to know and understand what factors affect the volume of gas produced, particularly as they relate to repeatability within or between laboratories. In studies involving gas pressure and volume measurement, the chemical composition of the culture medium, the nature of the microbial population and the type and amount of substrate available for fermentation all have a significant influence on gas accumulation. In this study, the manual pressure transducer technique (PTT) of Theodorou et al. (1994) was used to investigate the effect of head-space pressure on the resultant gas production profiles.