Twelve Holstein cows were matched into pairs after 7 days lactation. Each was fed 1 kg of grain concentrate per 3 kg milk and corn silage free choice. At 2 wk, one cow from each pair was maintained on this diet and the other was fed a protected tallow product mixed with the grain concentrate, 1 to 2, providing 25% of metabolizable energy as fat. For 14 wk, milk fat production was 12.4% higher and efficiency of utilization of metabolizable energy for lactation was 8.0 to 13.6% higher in tallow-fed cows. These had higher concentrations of glucose in plasma and cholesterol in serum and lower concentrations of acetoacetate and betahydroxybutyrate in plasma. At 16 wk, cows were fed in two groups, the concentrate to milk ratio was 1:4 and silage intake 41 kg/day. Digestible protein was reduced from 118 g/kg dry matter to only 80 g/kg. Milk production declined rapidly, so that duration of lactation averaged only 163 days in both groups. Protected tallow conferred metabolic advantages during the first 16 wk of lactation. Its influence on the duration of lactation requires further study in cows fed adequate protein.