Objectives were to evaluate the acute and chronic effects of electric heat blanket (EHB)-induced hyperthermia and feed restriction on immune cell dynamics and neutrophil function (as assessed by oxidative burst [OB) and production of myeloperoxidase [MPO]) in lactating Holstein dairy cows. Cows (n = 18) were allowed 4d to acclimate and then subjected to 2 experimental periods (P). Period 1 lasted 4 d during which cows were housed in thermoneutral conditions with ad libitum feed intake. During P2 (4 d), cows were assigned to 1 of 2 treatments: (1) artificially induced heat stress using an electric heat blanket (Thermotex Therapy Systems Ltd. Calgary, Canada) with ad libitum intake (HS; n = 10) or (2) thermoneutral conditions and pair-fed (PF; n = 8). Blood samples were obtained via jugular venipuncture on d4 of P1 and at 24 and 96h of P2. The EHB decreased DMI 32% on d1 and 52% on d4 of P2 in HS cows and, by experimental design, the pattern was similar in the PF cohorts. There were no treatment effects on circulating lymphocytes or monocytes. Circulating neutrophils from HS cows initially increased (35%) at 24 h but returned to baseline by 96 h, while they did not change in PF controls (P = 0.08). Band neutrophils progressively decreased in HS cows while they continuously increased in PF controls (P = 0.01). HS gradually decreased OB (6%, P < 0.01), while it initially decreased in PF controls (9%; P < 0.01) but returned to baseline by 96 h. Total MPO initially increased (13%; P < 0.01) in HS cows and then gradually returned to baseline by 96 h. Total MPO of PF cows increased (10%; P = 0.07) after 96 h of feed restriction. Overall stimulated MPO production increased (26%; P < 0.01) in HS cows, while it initially increased in PF controls (17%; P = 0.01) but it returned to baseline by 96 h. MPO exocytosis increased (20%) after 96h of HS while it initially increased (12%) in PF cows but it eventually decreased (14%) relative to baseline by 96 h. In summary, both feed restriction and HS discordantly influence circulating neutrophil dynamics. Whether these changes are reflective of “immune suppression” or rather immune activation are not clear.