Heat stress (HS) has detrimental effects on lactating cattle especially when they are in a negative energy balance. Feed additives have been shown to mitigate the effects of HS by improving metabolic and immune function. The objective of this study was to evaluate the effect of feeding a dietary supplement (PMI Nutritional Additives, Arden Hills, MN) on the HS response in multi-parturient dairy cows in mid lactation. Two pens of cows at a commercial dairy were fed either control (CON) or additive (FA) at 113 g/cow per d for 2 weeks before arrival. Study cows (n = 12) were balanced in days in milk (DIM), milk production, and parity (111.91 ± 4.85 d, 33.67 ± 0.96 kg/d, and 2.25 ± 0.18). Cows were randomly selected from both groups (6 TRT and 6 CON) and housed in environmentally controlled chambers for 18 d and fed appropriate diet. Cows were subjected to 7 of thermoneutral (TN) conditions, 7 d of HS, and 4 d of recovery (REC) under TN conditions. Dry matter intake (DMI), milk production, and milk composition were measured daily. Rectal temperature (RT) and respiration rate (RR) were measured at peak temperature daily. Blood samples were collected once daily at 1500 h following catheterization on d 4 of TN to d 4 of REC. Serum samples were analyzed for glucose, insulin, blood urea nitrogen (BUN), β-hydroxybutyrate (BHB), and nonesterified fatty acids (NEFA). Results were analyzed using repeated measures in the PROC MIXED of SAS. HS increased RT (P < 0.0001), RR (P < 0.0001), BUN (P < 0.0001), insulin (P = 0.04), neutrophil (P = 0.009), and water intake (P = 0.0005). HS decreased lymphocyte (P = 0.0008), DMI (P = 0.0007), energy corrected milk (ECM, P = 0.01), and 4% fat corrected milk (FCM, P = 0.02). FA decreased the feed efficiency ratio (P = 0.03). FA had no effect on blood parameters. There was a treatment x environment interaction with cows fed FA having lower feed efficiency (P = 0.02) during peak thermal loads than CON. Results of this study suggest that HS exposure had performance and metabolic impacts in mid lactation cows. Supplementation with FA alleviated some of the performance effects associated with HS.