The supplementation of dairy cows with yeast probiotics may increase diet digestibility, plasma niacin concentration, heat dissipation, and lactation performance. We evaluated the response of Holsteins in late lactation (234 ± 91 DIM) to dead yeast culture (YC. Fifteen g/d, Factor SC, Grasp. Saccharomyces cerevisiae) during Brazilian summer (THI >68 for 92.2% of time). Thirty-two cows were individually fed a standard TMR for 14 d and treatments Control (CTL) or YC for 35 d, in a covariate adjusted randomized block design. Response was evaluated in wk 5 or as repeated measures over time. Cows were milked 3x/d and treatments were orally dosed to each cow after each milking. Plasma niacin was 1.50 and 1.66 µg/mL on CTL and YC, respectively (P = 0.02). The YC reduced rectal temperature, respiratory rate, and body surface temperature (P ≤ 0.05) and tended to increase the sweating rate (P = 0.13) evaluated twice per week at 0730, 1500, and 2200 h. The proportion of cows with rectal temperature ≥39.2°C on CTL and YC was, respectively: 8 and 0% at 0730 h, 52 and 25% at 1500 h, and 35 and 26% at 2200 h (P ≤ 0.05. GENMOD of SAS). Plasma glucose in wk 3 and 5 were increased by YC (P = 0.01). The total-tract apparent digestibility of nutrients, plasma urea-N concentration, ruminal VFA profile, urinary allantoin excretion, ingestion and chewing behavior, and the proportion of daily intake in the morning, afternoon, and night were not affected by treatment (P ≥ 0.16). Cows on CTL rejected feed particles >19 mm in the morning (P = 0.02), in the afternoon were less selective against long feed particles (P = 0.05) and in favor of particles <8 mm (P = 0.03), and did not sort against short particles at night (P = 0.05). Milk yield was 30.5 kg/d for CTL and 30.2 for YC (P = 0.58). The YC reduced DMI (19.4 vs. 20.3 kg/d. P = 0.04) and increased the milk to DMI ratio (1.64 vs. 1.50. P = 0.05). Milk solids yield and concentration, linear SCC score, and MUN did not differ (P ≥ 0.21). The YC increased plasma niacin concentration, heat dissipation, and feed efficiency of late lactation dairy cows.