Apparent digestibility coefficients and serum parameters were measured to evaluate the effect of supplementing feed concentrates with rice bran oil in horses. Twelve horses (6 males and 6 females) with a mean age of 18 ± 4 months old and mean live weight of 306 ± 22.6 kg were used. Treatments consisted of increasing rice bran oil concentrate levels of 0, 3.5, 7.0, 10.5, 14.0 and 17.5%, considering a daily intake of 2.25% live weight on a dry matter basis. A dietary effect of supplementation on the apparent digestibility of gross energy (y = 64.55 - 0.58x) was observed (P<0.05). No effect on the other nutrients was observed (P>0.05). Supplementation did not affect serum glucose levels (P>0.05), but cholesterol was affected (P<0.01) based on the interaction of the treatment x gender parameter in females (y = 73.92 + 2.08x). However, no effect was observed in males (P>0.05). A dietary effect on the triglyceride (y = 15.73 - 0.96x + 0.0524x²) and HDL (high-density lipoprotein) (y = 45.24 + 1.0499x) parameters was observed (P<0.01). While the use of rice bran oil does affect blood parameters associated with lipid metabolism, rice bran oil levels up to 17.5% concentrate do not negatively affect the apparent digestibility of dietary nutrients.