To determine the effect of the physical form of ration on energy metabolism in ruminants, this experiment was conducted through evaluating gas exchange and energy cost associated with feeding behavior in goats fed rye hay through a respiration trial. Unchopped and chopped hay 2 cm in length at the boot and heading stage were offered ad libitum to twelve goats. The O₂ consumed, CO₂ and CH, produced, and heat production were slightly higher in groups fed unchopped hay than in groups fed chopped hay at either the boot or heading stage. Diurnal changes of O₂ consumed, CO₂ and CH, produced, and heat production exhibited the similar patterns in all groups, reaching a peak from 12 hr to 19 hr. On the other hand, O₂ consumed was not correlated with voluntary intake of dry matter (VDMI), digestible dry matter (VDDMI) and digestible energy (VDEI) but CO₂ and CH4 produced were closely correlated (P$lt;0.05 or 0.01) with VDMI, VDDMI and VDEI. Heat production was closely related (P$lt;0.05) only to VDEI. Energy cost of eating was higher in groups fed chopped hay than in groups fed unchopped hay at both stages, although there were no significant differences. However, that of rumination was greater in groups fed unchopped hay than in groups fed chopped hay. Energy cost per minute was the lowest during eating and was higher for standing than for lying in rumination in all groups. There was more spendable energy in rumination and resting than in eating and this was greater in groups fed hay at boot stage than in groups fed hay at heading stage. This can be interpreted to mean that the influence of the physical form of hay on energy metabolism is greater for low quality hay than for high quality hay.