Two changeover treatments were studied over a 5-week period in eight early-weaned concentrate-fed Holstein calves of initial weight 160 kg. Fresh forage was given ad libitum for one week (A) or two weeks (B); calves on the latter treatment also had 500 g daily of maize meal. All calves received 300 g/100 kg live weight of a protein supplement and had free access to sodium and phosphorus-rich minerals. A rumen inoculation was given after seven days of forage feeding, at which point a liquid molasses/urea mixture was introduced, first in small amounts and then after a further seven days ad libitum. The forage was then restricted to 3 kg/100 kg live weight. Both treatments were effective in raising rumen pH and buffering capacity to the point of allowing the establishment and maintenance of protozoa, chiefly Entodinium and Isotricha spp. Protein N as a percentage of the total N of rumen contents was about 90 with the all-concentrate diet, fell to 35 with the roughage diets and then rose again to 80–90 by the end of the experiment.