Four primparous Holstein cows were used in a 4 × 4 Latin square with 21-d periods to determine the effect of redfish oil and monensin sodium on milk composition. The four dietary treatments were a basal diet (control), the basal diet plus 14.5 mg monensin kg−1 dietary dry matter (M), 2% fish oil (FO), and a combination of fish oil and monensin (FO + M). Total DM intake, measured during the last week of each period was reduced on the two fish oil treatments but an interaction with monensin depressed intakes further. An additive inhibition of rumen fibre degradation is discussed. Differences in yields of milk and lactose were not shown to be significant among treatments. Milk fat content was reduced 29.8% by fish oil supplementation and protein content dropped 5.3%. Protein:fat ratios increased from 0.78 on the control ration to 1.08 on the fish oil treatments. Monensin, on the other hand, only caused a change in milk fat percentage, and that by a 7.5% decline. Fish oil increased the concentrations of 20- and 22-carbon fatty acids in milk, including the n-3 eicosapentaenoic and docosahexaenoic acids which were transferred at 9.3 and 16.2% efficiency from the diet, respectively. Monensin had no effect on milk fatty acid profile. The fish-oil-by-monensin interaction produced the poorest energy intakes and rates of energy deposition into milk. Key words: Milk composition, fish oil, monensin, omega-3 fatty acids