Two experiments were conducted to evaluate the effects of wheat straw treated with alkali on ruminal function and lactational performance of dairy cows. In Experiment 1, four ruminally fistulated Holsteins cows (X = 57 d of lactation) were fed four diets that contained 0, 20, 30, or 40% (dry basis) wheat straw treated with 3% NaOH plus 3% Ca(OH)2 in a 4 x 4 Latin square. The diets contained 60, 40, 30, and 20% (dry basis) prebloom alfalfa haylage to result in a 60: 40 ratio of forage to concentrate. In Experiment 2, 12 (X = 149 d of lactation) midlactation Holstein cows were fed diets containing 0 or 20% treated wheat straw to measure lactational performance during a 12-wk period. In Experiment 1, no effect of diet was observed on ruminal pH, osmolality, digestion of neutral detergent fiber of straw, or 4% fat-corrected milk. However, the dry matter intake of cows fed diets containing the two highest concentrations of treated straw was depressed and was associated with a significant body weight loss. The ruminal rate of passage of treated straw was not affected by diet; however, rate of passage of alfalfa haylage decreased when straw was included in the diet. In Experiment 2, no effect on cow performance was observed when straw treated with alkali was included at 20% of the diet. Inclusion of up to 20% wheat straw treated with 3% NaOH plus 3% Ca(OH)2 in diets of lactating cows resulted in ruminal function and performance that were similar to those of cows fed diets that contained alfalfa haylage only.