The aim of the study was to estimate how much endogenous urea nitrogen (EUN) was incorporated into various amino acids of milk protein when goats were fed low (LP), medium (MP), or high protein (HP) diets in a 3 × 3 Latin-square design. Three Alpine goats of about 35 kg body weight fitted with a catheter into the jugular vein were fed isoenergetic diets containing 11% (LP), 13% (MP), or 16% (HP) crude protein in dry matter. They were continuously infused with ¹⁵N urea into the jugular vein for 6 days. Daily milk yield was 1.58, 1.49 and 1.77 g. Milk urea concentrations were 139, 342 and 451 mg · l^–1, whereas plasma urea was 178, 356 and 667 mg · l^–1 in groups LP, MP and HP, respectively. Samples of milk protein were hydrolysed with 6 M HCl, and then free amino acids were converted into butyl derivatives using HCl in butanol, followed by N-acylation using trifluoroacetic acid anhydride. The amino acid derivatives were analysed using a gas chromatograph equipped with a mass detector. ¹⁵N-excess after a six-day infusion of labelled urea was significantly higher (P < 0.05) in the vast majority of amino acids of milk protein from goats fed the LP diet in comparison with goats fed the HP diet. Therefore, the protein level of diets affects the incorporation of EUN into amino acids of milk protein. EUN was incorporated primarily into glutamic acid, methionine and arginine. At all levels of nitrogen in the diets, the incorporation of ¹⁵N into phenylalanine was very low.