The reference values of the serum and urine biochemical parameters have not been well-studied in donkeys. This study aimed to assess the normal values of the selected renal biomarkers, such as: serum creatinine, blood urea nitrogen (BUN), albumin, total protein (TP), electrolytes and symmetric dimethylarginine (SDMA), urine protein concentration (UPC), urine protein/creatinine ratio (UPCR), the urine gamma-glutamyl transferase (GGTP)-to-creatinine ratio, serum creatinine-to-urine creatinine ratio (sCr/uCr), serum BUN-to-serum creatinine ratio (sBUN/sCr) and UPC-to-TP ratio, as well as the fractional electrolyte excretion of sodium (FENa) and potassium (FEK) in donkeys. The effects of age, gender and deep freezing of the serum material were investigated. Sixty-five healthy adult donkeys were involved in this study. The results showed higher BUN and TP values and lower albumin, UPCR, FENa and FEK levels in donkeys when compared to the reference values in horses. A significant gender relationship for creatinine and BUN was found. Age influenced the values of albumin, TP, potassium and chlorine. Potassium, sodium and SDMA did not show significant concentration changes after freezing. The study results demonstrated that horse reference range values for some parameters cannot be applied to donkey samples. Only a few of the serum parameters were not affected by freezing, and this should be taken into account when storing biological materials.