To study the effect of weaning age on average daily gain (ADG), dry matter intake (DMI) and ruminal fermentation, 10 rumen-cannulated male Holstein calves were randomly assigned to one of two treatments: (i) early weaned at 8 weeks of age (235 l milk); (ii) conventionally weaned at 12 weeks of age (347 l milk). Twice daily grass hay (9.0 MJ ME x kg(-1) DM) and a dry total mixed ration (TMR) (11.6 MJ ME x kg(-1) DM) containing 15% alfalfa hay and 85% concentrates were offered separately. Water was available ad libitum. Ruminal fluid was collected via cannulas at weeks 9, 11, 13 and 15, twice weekly just prior to as well as 1, 3, 5 and 7 h after morning feeding. Calves of both treatments achieved adequate ADG (947 vs. 959 g; p > 0.05). Just-weaned calves rapidly increased DMI (1.1-2.5 kg TMR and 2.4-3.6 kg TMR for early- and conventionally-weaned calves, respectively). From weeks 10-12 early-weaned calves consumed significantly more dry feed than conventionally-weaned calves (week 10: 2.5 vs. 1.6 kg/d; week 12: 3.4 vs. 2.4 kg/d). Early weaning stimulates DMI supporting ruminal fermentation intensity, indicated by lower ruminal pH. After weaning, only early-weaned calves achieved critical average ruminal pH (week 9: 5.7 vs. 6.0, p = 0.017; week 11: 5.9 vs. 6.2, p = 0.007). Experimental treatment did not affect the concentration of ruminal short-chain fatty acids (SCFA). For all calves, the effects of the concentrate-rich TMR were shown by a high SCFA level (daily average: 137-152 mmol x l(-1)) and an acetate to propionate to butyrate ratio between 51:36:9 and 54:33:10.