Two experiments were conducted to estimate the amount of muscle tissue and crude protein (CP) in carcasses and in empty body (EB) weigh from urinary creatinine excretion (UCE). In experiment I thirty-two Nellore bulls were used with an initial body weight of 259±24.9 kg and 14±1 mo old, as follow: four bulls were fed at maintenance level and the twenty-eight bulls had ad libitum access to feed throughout the experiment. The diet consisted of corn silage and concentrate (55:45). Three days before each slaughter, total urine was collected from all bulls within a slaughter group. After harvest, a complete dissection (fat, lean and bone) of the left half of the carcass was conducted and subsampled. Sub samples were lyophilized, partially defatted and ground for further evaluation. In experiment II fifteen Nellore heifers were allocated by body weight into three groups: 127±24.2 kg (low), 221±22.3 kg (medium) and 434±29.2 kg (high). Heifers were fed a diet consisting of 60% of corn silage and 40% of concentrate. After thirty days of adaptation, total urine was collected over a three day period using Folley catheters. After urine collection, the animals were slaughtered to estimate the physical composition of the carcasses and EB. The amounts of muscle tissue present in the carcasses of Nellore bulls and heifers (Mcarc) was estimated using the following equation: Mcarc (kg)=14.40×UCE (g/d). The amount of CP in the carcasses (CPcarc) and the empty bodies (CPEB) of Nellore bulls was estimated by: CPcarc=3.85×UCE (g/d) and CPEB=5.86×UCE (g/d), respectively. It was concluded that each gram of UCE corresponded with 14.40 kg of muscle tissue, 3.85 kg of CP in the carcass and 5.86 kg of CPEB.