A study on the excretion dynamics of urea nitrogen (NU) in endogenous urinary nitrogen (EUN) was carried out on 350 male Wistar rats in 14 age groups (k = 14), ranging from 23 to 273 days, with an average body mass (SB) from 39 to 446 grams. The rats in each k - group were randomly assigned to one of five subgroups, differing in the duration of the protein - free diet (PFD) feeding, t = 3, 6, 9,12 and 16 days. Urine for EUN and NU determination was collected during the entire experiment. The age of the rats (k) was found to have a significantly greater influence on the concentration of NU and EUN than the duration of the PFD feeding (t). The interaction of k ∙t was significant. The proportion of NU in EUN increased distinctly in successive t periods to age of about 80-100 days, after which it rose only slightly in older rats. A distinct drop in the proportion of NU in EUN was seen between the period of 3 and 16 days of feeding the PFD. NU excretion (mg) as a function of urinary EUN excretion (mg) and duration of the PFD feeding described by the linear multiple regression equation after transformation to power form is given below: NU = 0.27∙ EUN^1.187 ∙ t^-0.185 (1) Daily urea N excretion (NUd) was highly correlated with body mass (SBd), increasing with age and decreasing with the duration of feeding the PFD. This relationship describes the multiple regression equation of which exponential transformation is given below: NU_d = 225.4 • SB_d^1.033 (in kg) ∙ d^-0.242 (2) Equations (1) and (2) make it possible to obtain a satisfactory estimate of the amount of endogenous NU in the urine of rats fed for 16 days on PFD and the daily urea N (NU_d) excretion on days 1 to 16 on a PFD.