Urinary protein excretion occurs in neonates of many animal species, as well as in human neonates. However, the incidence, dynamics, and mechanism of proteinuria have not been unambiguously explained. The aims of this study were to investigate into excretion of selected protein fractions of molecular weight less than 69 kDa (LMW), evaluation of intensity and dynamics of changes during the first month of kids' life, and an attempt to explain the causes of neonatal proteinuria. The analysis were carried out on 16 kids of White Improved goats, over the period from birth until 30 days of age, using clearance methods. Urine proteins were separated electrophoretically (SDSPAGE), and their concentration and percentage content was determined by densitometric method with the use of archiving and image analysis software. The proteins found in the urine were grouped as HMW, LMW and albumin. For six fractions of LMW proteins, excretion rates and percentage content of the urinary total LMW protein pool were calculated. It has been demonstrated that neonatal proteinuria in goat kids is associated with a high level of excretion of proteins of lower molecular weight than albumin (69 kDa). A strong dynamics of changes in excretion of particular LMW protein fractions with age was observed, which may imply not only an increased permeability of glomerular filtration barrier, especially over the first days of life, but also a selectivity of reabsorption mechanisms in the nephrons. An increased permeability of glomerular filtration barrier for proteins during the first days of life may represent the adaptive mechanism for removal of protein excess from the organism. The urinary LMW protein pool may also contain proteins resulting from the hydrolysis in the tubular cells.