Experiments were conducted with 1-3 years old, rumen cannulated Merino wethers to characterize the stability of the 1,4-β-endo-xylanase activity of a commercially available enzyme preparation (Rumino-Zyme, 250 FXU/g xylanase activity, Dr. Bata Ltd., Hungary). The Remazol blue technique used for determination of the xylanase activity of the rumen fluid and solid samples proved simple, quick and reliable. The xylanase activity in the rumen of fasting wethers was found to be 20-30 FXU/l. It was by about 2.5 times greater two and four hours after feeding than the value measured pre-feeding. Exogenous xylanase added directly into the rumen of fasted wethers caused an increase in the enzyme activity. After the peak activity at 5 min after treatment the enzymatic activity decreased with an estimated half-life of 45-50 min. This result of the present investigation did not support the earlier observations, which indicated lasting activity of external NSP-enzymes in vivo.