The biochemical origin and the physiological significance of the urinary nitrogenous end products as the basis for the determination of the habitat adaptation of adult anurans are discussed. The analysis of the available information and the original data produced in the author`s laboratory have shown that neither the profile of N-end products found in the urine nor the liver arginase activity are reliable differential parameters to determine the aquatic (ammonotelism) or terrestrial (ureotelism/uricotelism) fitness of a particular species. It is suggested that more consistent discrimination of the habitat adaptation of amphibians could be obtained measuring plasma urea-N, plasma/urine urea-N concentration ratios and the activities of selected liver enzymes involved in purine biosynthesis and uric acid breakdown.