The effect of a hydrogenated fish oil (HFO; Zamrog Polienergía ®) on milk production, fat and milk protein contents, dry matter intake and body weight were evaluated in 30 Holstein Friesian lactating multiparous cows with over 9,000 liters per lactation and somatic cell counts below 400,000· ml-1 per quarter. Cows were randomly distributed in three groups of ten individuals each. All diets had the same energy (2.8- 2.9 EM·Kg MS-1), crude protein (17- 18 %) and neutral detergent fiber (28-32 %) levels. HFO was added at rates of 0.0, 3.0 and 6.0 % on a dry matter basis. The trial began after an adaptation period of two weeks postpartum and continued for 15 weeks. Feed consumption per group was controlled weekly, as well as milk production and body weight. Samples of milk were taken for total solids, fat and milk protein, every two weeks. Milk production increased significantly (p<0.05) in groups with HFO; in group 3, this reached 8.78% more than the control group. The same group showed 3.4 l higher at peak milk production than the control group. Fat and milk protein did not show significant differences (p<0.05) among groups. No significant differences in dry matter intake were observed among groups. The largest body weight losses were reached within two weeks postpartum and the control group presented the highest losses. The group fed 3% protected fat lost the least weight despite the lowest dry matter intake, thus suggesting a greater efficiency in energy utilization resulting in lower fat mobilization and a higher peak yield and milk production during the trial. These results show that fish oil hydrogenated fat does not disturb the ruminant function and, it is a good way to increase energy concentration of the ration and thus milk and fat milk production, without decreasing the remaining milk nutrients.