Rapid rumen starch fermentation of wheat increases the heat of fermentation and may increase the risk of heat stress (HS). Therefore, feeding slowly fermentable grains such as maize and reducing metabolic heat load may reduce HS in grain-fed sheep. Twenty-two Merino x Poll Dorset wethers were housed in 2 climate-controlled rooms and were fed either maize grain plus forage (39% starch) (MF, n = 11) or wheat grain plus forage (37% starch) (WF, n = 11) during 3 experimental periods: P1) 7 d of thermoneutral conditions (18–21°C and 26–30% relative humidity (RH)) and restricted feed intake (85% of unrestricted feed intake) ; P2) 7 d of HS (28–38°C and 40–50% RH) and restricted feed intake ; and P3) 7 d of HS as P2 with unrestricted feed intake (1.5 times maintenance) in a complete randomized block design. Water was offered ad libitum. Physiological parameters—rectal temperature (RT), heart rate (HR), respiration rate (RR) and left and right flank skin temperature (LST, RST)—were measured at 0900, 1300, 1700, and 2100 h. Blood samples were collected on d 2 and 7 of each period for pH, blood gas and hematocrit (HcT). Data were analyzed using restricted maximum likelihood (REML) in GenStat v14. All physiological parameters were elevated (P < 0.001) during HS especially during P3. RR, RT, LST, RST and HR were lower (P < 0.05) in sheep fed MF, particularly during HS. LST was higher (P < 0.001) than RST and sheep feed WF had a larger (P < 0.001) difference between LST and RST. During HS total CO₂ (cTCO₂), hemoglobin (cHgb), bicarbonate (cHCO⁻³), HcT, lactate, glucose and base excess were lower than P1 (P < 0.05). In sheep fed MF, the partial pressure of CO₂, cTCO₂ and cHCO⁻³ were higher (P < 0.05), while cHgb, pH and HcT were lower (P < 0.05) than WF. In conclusion, dietary maize, a slowly fermentable grain, reduced the metabolic heat load from feeding, expressed as reduced physiological parameters at thermoneutral conditions and ameliorated some of the physiological responses negatively affected by HS compared with dietary wheat.