Voluntary feed intake, digestibility and rumen flow- and fermentation dynamics were studies using sheep. NaOH treated (5 % w/w) wheat straw, washed and. unwashed as well as untreated straw were compared. Results indicated that washing the straw had little effect on in vivo digestibility, voluntary feed intake. rate constants for fermentations and rate constants for outflow, but water intake and the turnover time of a water soluble marker (Cr- EDTA) in the rumen and the percentage fermentable  O.M. in the rumen were significantly influenced. When compared to untreated straw, NaOH treatment appeared to increase the total in vivo fermentability (α) and rate of organic matter (O.M.) outflow. The.se seemed to be responsible for the increased voluntary intake of digestible O.M. often observed with NaOH treated forages. The rate constant for fermentation, expressed as a first order reaction rate constant, appeared to be unaffected by NaOH treatment of these diets.