Effects of Saccharomyces cerevisiae fermentation product (SCFP; Original XPC) on rumen volatile fatty acid (VFA) and lipopolysaccharide (LPS) concentrations during heat stress were measured in a crossover experimental design consisting of 2 28 d-periods. Eight cannulated Jersey cows (nonlactating, non-pregnant) were housed in tie stalls containing individual feed bins and an automatic watering system. Cows were fed a mixture of chopped grass hay and a grain mix twice daily and 1 of 2 treatments. Treatments consisted of 14 g Control (grain mix) or 14 g SCFP and were fed before the morning feeding. The THI of the barn was maintained at 80 to 83 during the trial. Rumen fluid samples were collected from 5 locations within the rumen (cranial dorsal, cranial ventral, central rumen, caudal dorsal, and caudal ventral) every 3 h from 0900 h to 2100 h on d 26, 27 and 28, composited within cow, and strained. Samples were analyzed for VFA and LPS concentrations. Data were analyzed using the fit model procedure of JMP. Cows supplemented with SCFP had greater (P < 0.0001) concentrations of total VFA, acetate, propionate, and butyrate than control cows (103.46 vs. 95.19; 69.14 vs. 63.78; 18.75 vs. 17.21; 12.16 vs. 10.80 mM, respectively). Molar proportions of acetate and propionate were not affected by treatment. Molar proportion of butyrate was higher (P < 0.002) for cows supplemented with SCFP than control cows (11.72 vs. 11.32%, respectively). Molar proportions of valerate, isobutyrate and isovalerate were lower (P ≤ 0.007) for cows supplemented with SCFP (1.01 vs. 1.08; 1.18 vs. 1.27; 1.11 vs. 1.20%, respectively). Although not significantly different, cows supplemented with SCFP tended to have lower trend (P = 0.16) LPS concentrations than control cows (17,327 vs. 20,156 endotoxin units). Results show that SCFP can maintain better rumen function in cows subjected to heat stress as indicated by higher rumen VFA concentrations. This effect could be the result of stabilized rumen microbial populations, as indicated by the reduced tendency of LPS concentrations in the rumen.