The objective of this paper was to assess the plasma antioxidant status ewes under high ambient temperature fed seaweed Ascophyllum nodosum (AG), flaxseed (FS) and their combination (FS + AG) measured by the Antioxidant/Oxidant Balance (AOB). The experiment lasted 30 day and involved thirty-two late-lactation Comisana ewes, divided into four balanced groups of eight each, and exposed to high ambient temperature. The ewes in each group were individually fed twice daily and received 1.8 kg per day of oat hay. Results showed that ewes fed the combination of flaxseed and A. nodosum displayed the highest AOB value at day 30 of high ambient temperature exposure; moreover, they displayed lower rectal temperature than ewes fed A. nodosum alone, and lower concentrations of heat shock protein 70. These data demonstrated that supplementation of diet with A. nodosum in combination with flaxseed can protect ewes from the excessive oxidative imbalance caused by high ambient temperature.