The study presents a statistical analysis of methane production by sheep and cattle, based on over 2500 daily determinations. Methane production was measured using closed-circuit respiration equipment, and the results were analyzed to determine the relationship between methane production and diet. The study found that methane production varied significantly between individuals, with a coefficient of variation of about ±7%. The daily variation in methane production was also significant, with a coefficient of variation of ±7.2%. The study also found that methane production was related to the apparent digestibility of the diet. At the maintenance level of feeding, methane production (Cm) varied from 6.2 to 10.8 kcal/100 kcal feed and was related to the apparent digestibility (D) of the diet by the equation Cm = 3.67 + 0.062D. The regression was statistically significant, with a residual standard deviation of ±0.71 or ±8% of the mean amounts. The change in methane production when feeding level was increased from maintenance to 2× maintenance (b) was also related to the apparent digestibility (D) of the diet by the equation b = 0.050D - 2.37. This equation was also statistically significant. The study also found that as the apparent digestibility of the feed increases, methane production, expressed as kcal/100 kcal feed, increases markedly at the maintenance level of feeding, but at feeding levels of 3× maintenance it falls. The study concluded that the relationship between methane production and diet is complex and that previous estimates based on dry matter consumption or total digested carbohydrate may be inaccurate. The study provides a new equation for predicting methane production based on apparent digestibility and feeding level.The study presents a statistical analysis of methane production by sheep and cattle, based on over 2500 daily determinations. Methane production was measured using closed-circuit respiration equipment, and the results were analyzed to determine the relationship between methane production and diet. The study found that methane production varied significantly between individuals, with a coefficient of variation of about ±7%. The daily variation in methane production was also significant, with a coefficient of variation of ±7.2%. The study also found that methane production was related to the apparent digestibility of the diet. At the maintenance level of feeding, methane production (Cm) varied from 6.2 to 10.8 kcal/100 kcal feed and was related to the apparent digestibility (D) of the diet by the equation Cm = 3.67 + 0.062D. The regression was statistically significant, with a residual standard deviation of ±0.71 or ±8% of the mean amounts. The change in methane production when feeding level was increased from maintenance to 2× maintenance (b) was also related to the apparent digestibility (D) of the diet by the equation b = 0.050D - 2.37. This equation was also statistically significant. The study also found that as the apparent digestibility of the feed increases, methane production, expressed as kcal/100 kcal feed, increases markedly at the maintenance level of feeding, but at feeding levels of 3× maintenance it falls. The study concluded that the relationship between methane production and diet is complex and that previous estimates based on dry matter consumption or total digested carbohydrate may be inaccurate. The study provides a new equation for predicting methane production based on apparent digestibility and feeding level.