Wheat straw, barley straw, alfalfa hay, alfalfa silage and maize silage were offered ad libitum to three male sheep, and daily dry matter intake (DMI) and apparent dry matter digestibility (DMD) were measured. Samples of forages were incubated with rumen fluid to determine gas production. Gas productions were measured at 3, 6, 12, 24, 48, 72 and 96, and gas production constants (a, b, c,) were described using the equation y = a + b (1 - e-(ct)). Gas production at all incubation times and gas production constants (c, b and a+b) were significantly (P < 0.001) correlated to DMD or DMI. Although the gas production constant c explained 74% of the variation of DMD, the gas production constant b explained 92% of the variation of DMD. The gas production constants c and (a+b) in combination explained 96% of the variation of DMD. The gas production constants b and c alone explained 70% and 78% of the variation of DMI respectively. The gas production constants in combination explained 84-90% of the variation of DMI. It was concluded that the accuracy of predicting DMD or DMI can be increased when gas production constants are used in combination, and the in vitro gas production technique has good potentiality to predict DMD and DMI.