Methane emissions from ruminant livestock in New Zealand (NZ) represent 31.8% of New Zealand's total greenhouse gas emissions and accurate inventories of methane production for all farmed livestock classes are required by the Kyoto Protocol. Currently, methane yields used to calculate total ruminant enteric methane inventories for adult (greater than one year of age) sheep (20.9 g CH4/kg DMI), dairy cattle (21.6 g CH4/kg DMI) and deer (21.5 g CH4/kg DMI) appear to be similar. However, the value used for deer is an average of adult sheep and dairy cattle, gained from actual methane measurements in those species. This is despite possible differences in digestive physiology and apparent digestibility. These findings are also confounded by methane measurements of each species being taken at different times with different diets. The aim of this study was to directly compare the methane yields of cattle, sheep and red deer when animals were fed the same diet of ensiled lucerne (Medicago sativa). Methane measurements were taken at the same time from animals housed under similar conditions and to compare differences in seasonality. An experiment to measure methane production and yields from non-lactating dairy cows (cattle) (n = 11), sheep (n = 11) and red deer (deer) (n = 11) was conducted at AgResearch Grasslands (sheep and cattle) and Massey University (deer), Palmerston North, NZ, in summer (January 2007) and winter (June 2007). Each measurement period was conducted over two weeks, of which the first ten days consisted of an adaptation period to the diet and housing, followed by four days of methane and DMI measurements. All animals were individually housed in metabolism cages to allow for individual DMI to be measured directly. Animals were fed ensiled lucerne chaff 'Chaffhage' to ensure consistency of diet between species and across seasons, at a rate of 1.2 times estimated energy requirements for maintenance of each species. Calculations used for energy requirements were based on the Australian feeding standard for ruminants. Methane measurements were made using the SF6 technique and collection equipment used was modified to suit each species. The preliminary results of this study are presented in Table 1. Dry matter intake per kg of metabolic live weight was greater for cattle (0.060 kg DMI/kg LW0.75) compared with deer (0.046 kg DMI/kg LW0.75) and sheep (0.048 kg DMI/kg LW0.75) (P<0.0001). Methane production of cattle (140.4 g CH4/day), was greater compared to deer (31.5 g CH4/day) and sheep (18.3 g CH4/day) (P<0.0001), and deer produced more methane than sheep (P<0.005). Methane yields also differed with species, cattle (20.6 g CH4/kg DMI) > sheep (18.4 g CH4/kg DMI) > deer (16.5 g CH4/kg DMI) (P<0.0003). An interaction of species by season of measurement for both methane production and yield was found, which appeared to be due to the increase in methane production and yield of cattle from summer to winter (P<0.0002). There was found to be no effect of season on of methane yields of deer or sheep. The preliminary findings of this experiment show that there is a difference between ruminant species in methane emissions when measured at the same time on the same diet. This could mean that the current inventory value for deer, which is an average of sheep and cattle, overestimates total methane yields. Differences in the digestive processes and physiology or microbial populations between ruminants may be responsible for the differences in methane yields. Measurements of apparent digestibility, rumen fermentation, digesta flow and microbial populations have also been taken and may help to explain the results presented here.