This research was performed to study the effects of fructo-oligosaccharide (FOS) and mannan-oligosaccharide (MOS) on nutrient digestibilities, fecal pH and volatile fatty acids compositions, and immune function in adult horses. Four adult Thoroughbred horses were used in a 4×4 Latin Square design with 20-day treatment periods. A 12-day adaptation phase was followed by 6-day collection of feces and 2-day blood sampling phases. Four different treatments were administered: 1) Control (no supplemental FOS or MOS; 2) 30 g of FOS/d; 3) 30 g of MOS/d; 4) 15 g of FOS+15 g of MOS/d. There were no differences among treatments in the fecal pH and volatile fatty acids compositions (P > 0.05). Nutrient digestibilities did not differ among treatments (P>0.05). Also, no differences were found in the IgA, IgG, and IgM levels among the treatments (P>0.05). Fructo-oligosaccharides, MOS, or FOS+MOS supplementation did not affect gut health and immune system in adult horses compared to control. © TÜBİTAK.