In a previous study in Reading (altitude 66 m) (Mauricio et al., 1997) the lag phase was greater when cow faeces was used as a source of microorganisms in the in vitro gas production technique instead of rumen liquor when twelve temperate forages were fermented for 96 h. In the Reading study faeces and rumen liquor were obtained from a cow fed grass silage and concentrate (60:40). The present study was done in Piracicaba, Brazil-BR (altitude 780 m) which has a tropical climate. Using the same forages as in Mauricio et al. (1998), the study examined whether the same differences between faeces and rumen liquor would occur in a tropical environment. In addition, the opportunity was taken to develop an equation relating pressure and volume for the semi-automated pressure transducer technique and compare it with the equation developed in UK by Mauricio et al. (1998).