The effects of supplements with different levels (high, medium, low) of ruminal undegradable protein (RUP) by Brown-Swiss heifers grazing Brachiaria brizantha cv. Marandu on the intake, digestibility and plasma urea nitrogen concentrations were evaluated. The experiment lasted 90 days, divided in three periods of 30 days, corresponding to May, June and July 2000. Fifteen purebred Brown Swiss heifers (five by treatment), averaging 19.7 months and 394 kg live weight (LW), were used. The pasture availability was monitored to maintain offer of 6% LW, in green dry matter (GDM). The heifers were fed 2.5 kg/head/day of concentrate supplement, offered twice a day. Extrusa was collected in animals fistulated in the esophagus. Indigestible neutral detergent fiber (NDF) in the feeds and feces was used to determine the intake and digestibility. There were no effects of supplements on the pasture, however there were differences among periods, because, as dry season progressed, the forage availability decreased and leaf/colm ratio and senescent material proportion increased. There was no effect of treatments on forage intake or forage + supplement. DM, organic matter (OM) and CP digestibility, and total digestible nutrients (TDN) content were lower for the diets with high RUP content. The animals fed supplement with high RUP content showed average plasma N-urea concentrations lower than those fed supplement with average RUP contents, followed by those fed diets with low RUP.