The objectives of this study were to characterize the establishment of ruminal fermentation and enzymaticactivities in dairy calves from birth to weaning (d 83).Six Holstein calves, immediately separated from their mother at birth, were fed colostrum for 3 d after birth,and thereafter milk replacer, starter pelleted concentrate, and hay until d 83 of age. Ruminal samples werecollected from each calf every day for the first 10 d, and additionally at d 12, 15, 19, 22, 26, 29, 33, 36, 40,43, 47, 50, 55, 62, 69, and 83. Ruminal samples were collected 1 h after milk feeding with a stomach tube.The pH and redox potential (Eh) were immediately measured. Samples were kept for further determinationof ammonia nitrogen (NH3-N) and volatile fatty acid (VFA) concentrations, and xylanase, amylase, urease,and protease activities. Ruminal pH averaged 6.69, 5.82, and 6.34, from d 1 to 9, d 10 to 40, and d 43 to83 of age, respectively. At first day of life, the ruminal Eh value was positive (+224 mV). From d 2 to 9, d10 to 40, and d 43 to 83 of age, ruminal Eh averaged −164, −115, and −141 mV, respectively. From d 1 to3, d 4 to 22, and d 26 to 83 of age, NH3-N concentration averaged 60.1, 179.8, and 58.2 mg/L, respectively.No VFA were detected in ruminal samples collected on d 1 of life of calves. From d 2 to 10 and d 12 to 83of age, ruminal total VFA concentration averaged 19.5 and 84.4 mM, respectively. Neither ruminal xylanaseor amylase activities were observed at d 1 of age. From d 5 to 15 and d 19 to 83 of age, the xylanase activity averaged 182.2 and 62.4 μmol of sugar released per hour per gram of ruminal content dry matter (DM), respectively. From d 5 to 83 of age, the amylase activity reached 35.4 μmol of sugar released per hour per gram of ruminal content DM. The ruminal ureolytic activity was observed with an average value of 6.9 μg of NH3-N released per minute per gram of ruminal content DM over the 83-d experimental period. From d 1 to 4 and d 5 to 83 of age, the proteolytic activity was 8.2 and 27.9 optical density units per hour per gram of ruminal content DM, respectively. The fermentative and enzymatic activities were rapidly established in the rumen from d 2 after birth. Most parameters did not evolve further after 1 mo of age.