We compared changes in the particle size profiles, permeability and elastic shear modulus of digesta in the forestomach and rumen of fasting tammar wallabies (Macropus eugenii eugenii) and fistulated sheep respectively that had been fed chopped lucerne hay. The wet mass of digesta in the tammar wallaby stomach declined curvilinearly over 24 h. The relative proportion of particles >2 mm in size in tammar wallaby digesta increased significantly and that of particles <2 mm in size decreased significantly after 12 h of fasting. This contrasted with the sheep rumen digesta, in which the relative proportions of coarse and fine particles did not change significantly over time. The permeability of wallaby digesta increased significantly after 24 h whilst that of sheep declined. All samples of tammar digesta had a significant elastic component (G′) that was preserved throughout the period of fasting. Interaction between component particles was significant at all times, digesta behaving as a weak gel. The ratio of energy lost to energy stored during flow of digesta tended to decrease during the period of fasting, indicating an increase in behaviour as an elastic solid. The relationship between G′ and dry matter content and mean particle size indicated that these phenomena resulted from progressive loss of finer digesta particles and that digestion in the wallaby stomach, via permeation of the particulate by the fluid phase, was possible for up to 33 h after eating.