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The study was designed to investigate the in situ degradability and in vitro digestibility of forage ingested by sheep grazing on pastures composed exclusively by annual ryegrass (Lolium multiflorum Lam., Treatment 1), black oats (Avena strigosa Schreb., Treatment 2) and mixed of both species (Treatment 3). Three crossbred Suffolk rams, 18 months age and 70 kg BW, were implanted with esophageal cannulas and used to collect the forage samples. The grazing method was continuous with variable stocking and grazing pressure was adjusted to maintain the forage supply in 7% BW in DM.d-1 . The three rams were allocated in the same treatment every three days and submitted to each treatment nine times. Aliquots from extrusa were analyzed for determination of DM, CP and NDF levels. Subsequently the samples were submitted to in situ degradability and in vitro digestibility analysis. The degradability of DM, CP and NDF was consistent to the characteristics and the developmental stage of the pastures, and were higher for ryegrass, intermediate for the mixed pasture and lower for black oats. Variations in characteristics of pastures and changes in behavior selection of forage by grazing animals determined high variability to in vitro digestibility of DM, CP and NDF. Higher values for in vitro digestibility DM and NDF were presented in annual ryegrass, which demonstrated the high quality of this specie compared to black oats in winter. The highest in vitro digestibility of CP in mixed pasture was associated with the presence of ryegrass at the vegetative stage and the black oats in the flowering stage on ingested forage. [ABSTRACT FROM AUTHOR]