Eight Holstein lactating cows (68 ± 3.46 d in milk) were assigned to 2 contemporary Latin squares in a 2 × 2 factorial design during the summer months. The 4 treatments were (1) no additive, (2) monensin supplementation at a rate of 30 mg/kg DM, (3) supplementation of 0.5 g/kg DM of a commercial blend of functional oils containing cashew nut shell liquid and castor oil as active ingredients (Essential, Oligo Basics Agroind. Ltda., Cascavel, Brazil), and (4) supplementation of both monensin and functional oils at the rates supplemented in treatments 2 and 3. Average temperature heat indexes ranged between 70 and 90. Whereas the inclusion of functional oils increased intake (15.30 vs. 16.67 kg; P < 0.001), monensin decreased it (16.89 vs. 15.57 kg; P < 0.001). Feeding functional oils increased 3.5% fat corrected milk (P < 0.001) but only when cows were not supplemented with monensin (interaction monensin × functional oils: P < 0.01). Also, the supplementation of functional oils increased both the protein and fat yield (P < 0.01) and the protein percentage in the milk (P < 0.01). Monensin supplementation improved milk production efficiency (P < 0.01) but decreased fat percentage (P < 0.05) and total solids in milk (P < 0.01). Only the supplementation of monensin changed the ruminal fermentation parameters, increasing propionate (P < 0.05), and consequently decreasing the acetate to propionate ratio (P < 0.05). Rectal and skin temperatures were not affected by any treatment. However, the respiratory frequency was increased by the supplementation of functional oils (P < 0.05). In conclusion, whereas the inclusion of functional oils improved the cows’ productive parameters without altering rumen fermentation, monensin supplementation decreased the amount of milk fat.