Capsaicin can promote peripheral blood flow and essential oils can have digestive and systemic action in mammals. We evaluated the effect of a blend of capsaicin, carvacrol, cinnamaldehyde, and eugenol (CEO. 176 mg/kg of TMR DM) relative to monensin (MON. Fifteen mg/kg of TMR DM) or control (CTL) on body homeothermy and systemic variables of dairy cows. Thirty individually fed Holsteins (189 ± 83 DIM) received a standard TMR for 14 d and a treatment for 56 d, in a covariate adjusted randomized block design with repeated measures over time. Pre-planned contrasts were: C1) CTL vs. CEO and C2) CEO vs. MON. Treatments had no effect on the concentrations of plasma D-lactate, BHBA, aspartate, and gamma-glutamyl transferases at 12-h post-feeding on d 25 and 53 and glucose on d 14, 25, 39, and 53 at 0, 4, and 12 h post-feeding (P ≥ 0.30 for C1 and C2). The CEO increased pO₂, the % oxygen saturation of hemoglobin, and the % oxygen of total volume in jugular blood on d 24 and 52 (P ≤ 0.03 for C1 and C2). On d 26 and 58, the diameter of the caudal artery evaluated by doppler-image ultrasonography tended to be larger on CEO than CTL (P = 0.08 for C1) and artery area was increased (P = 0.04 for C1). Blood flow and pulsatility did not differ among treatments (P ≥ 0.30 for C1 and C2). Rectal temperature (RT), skin surface temperature (ST), and respiratory frequency were measured at 3–4 d intervals (16 measures) at 8, 16, and 22 h. Cows on CEO tended to have lower RT at 16 h (P = 0.12 for C1), but did not differ at 8 h and 22 h (P ≥ 0.22 for C1 and C2). The proportion of cows with rectal temperature ≥39.2°C at 16 h was 18.7% for CEO and 30% for CTL and MON (P = 0.02), and at 8 h 10.4% and 19%, respectively (P = 0.02), with no treatment effect at 8 h (6%. P = 0.37) (GENMOD of SAS). The ST of CEO cows at 8 h was higher than CTL (P = 0.01 for C1) and tended to be higher than MON (P = 0.07 for C2). Respiratory frequency did not differ (P ≥ 0.43 for C1 and C2). The CEO increased the sweating rate (P = 0.05 for C1 and C2) and the evaporative heat loss (P = 0.01 for C1 and P = 0.12 for C2) measured once per week. The CEO increased heat dissipation and oxygen concentration in blood of dairy cows, showing to have systemic action.