Abstract: Factors determining the digestive efficiency of donkeys were studied in animals fed either a low quality roughage (wheat straw: 77{middle dot}1% neutral detergent fibre, 2{middle dot}8% crude protein) or a high quality forage (alfalfa hay: 47{middle dot}5% neutral detergent fibre, 22{middle dot}7% crude protein). The neutral detergent fibre (NDF) intake when fed wheat straw was 1693 {+/-} 268 g animal-1 day-1, 10% higher than when fed alfalfa hay. Digestive coefficient of NDF and acid detergent fibre (ADF) when fed wheat straw amounted to 50{middle dot}9 {+/-} 4{middle dot}9 and 42{middle dot}0 {+/-} 4{middle dot}1% respectively. NDF and ADF apparent digestibilities and mean retention times (37{middle dot}7 {+/-} 1{middle dot}7 and 36{middle dot}4 {+/-} 3{middle dot}2 h respectively) were not significantly different (P [rang] 0{middle dot}05) between the two diets. The donkey appears to digest cell wall constituents as efficiently as the Bedouin goat when on low quality roughage, but less efficiently when fed alfalfa hay. Its energy digestibility is, however, as high as that reported for the Bedouin goat. The donkey's high energy digestibility is related to its capacity to digest soluble food components more efficiently than the ruminant. The mean retention time in the donkey is shorter than in the Bedouin goat and is consistent with its capacity to compensate for a lower quality diet by increasing its intake rate. Recycling of urea in donkeys maintained on wheat straw amounted to 75{middle dot}5 {+/-} 13{middle dot}0% of the entry rate. A decrease in the rate of renal urea filtration, coupled with an increase in the fraction reabsorbed, increased the retention of nitrogenous waste and permitted recycling of nitrogen into the gut. N1 -