After 3 wk on a standardization ration of alfalfa hay and corn silage ad libitum and concentrate at 1 kg/3 kg milk, 15 lactating Holstein cows were fed 2.3 kg hay/day and one of five concentrate rations ad libitum for a 6-wk experiment. Cows were returned to the standardization ration after the experimental period. Concentrate rations during experimental period were: 1) control, 2) 14% dried whole whey, 3) 5.9% high mineral whey product, 4) 11.8% demineralized whey product, and 5) 9.8% lactose. Ration 3 contained the same amount of minerals from whey as ration 2 while rations 4 and 5 contained the same amounts of lactose as ration 2. Whey products replaced portions of corn and soybean meal in the rations. Total protozoa numbers in rumen contents averaged 1.8 X 105 /ml and were not different among times although they tended to be less during the experimental period on ration 4. Dasytricba decreased on rations 1, 2, and 4, while trends in numbers of Isotricha, Entodinium, and Diplodinium were not consistent. Only a few Ophryscolex were in a couple of the rumen samples. Bacterial numbers increased from 4.0 X 109/ml during standardization to 5.8 X 109/ml during the high-grain period, then returned to 3.8 X 109/ml in the post-experimental period. The number of lactose fermenters increased on all diets containing whey or whey products but not on the control diet. No differences in numbers of starch digesters were detected between times or among experimental rations because of large variations in numbers. Numbers of proteolytic organisms were not different among experimental rations but were generally lower during the post-experimental period.