The relationship between faecal nitrogen (FN), dietary nitrogen (DN) and dietary metabolizable energy (DE) was studied in two localities in the region of the Jeseníky Mts, Czech Republic, during four seasons. The content of nitrogen in plants significant for nutrition ranged between 0.99 and 3.86 g/kg of dry matter and DE was from 7.8 to 10.7 MJ/kg of dry matter. The DN/DE ratio in individual plant species ranged from 1:2.49 (stinging nettle) to 1:9.05 (rowan). The content of nitrogen in vegetation samples correlated with the content of DE in all four seasons (p < 0.01). In both the red and roe deer the diet contained more DN and DE during the vegetation season (spring to autumn) than in winter. The content of FN corresponded to the changes in diet quality and was positively correlated with DN and DE contents in all cases. On the basis of the correlations found between FN and diet quality we conclude that the use of FN for evaluation of diet quality in ungulates is possible, on the condition that we know the composition of their diet in the studied environment. However, it is unwise to compare different feeding specialists or individuals of one species living in different habitats with different diet composition.