Determining the urea concentration in milk is a useful indicator of the nutritional protein status of the organism as well as of the ratio between the energy and the protein in ruminant rations, with increasing practical usage. In addition to nutrition, milk urea concentration is influenced by a whole range of factors, for example: breed, stage and number of lactations, body weight, daily production and chemical composition of milk, somatic cell count, season and milking. The objective of this research was to determine the impact of the cow breed (Holstein and Simmental), the number of lactation (1st, 2nd, 3rd, 4th, 5th), milking time (morning-evening) and season (spring-summer and autumn-winter) on milk urea concentration. The following was determined for each breed: daily milk yield, milk fat, protein and lactose content, urea concentration and somatic cell count in milk. Statistical data processing was carried out by applying General Linear Model procedure, SAS system (1999). The cow breed had a significant influence on daily milk yield and log somatic cell count (P<0.001), lactose content in milk (P<0.01), milk fat content and milk urea concentration (P<0.05). The number of lactations significantly influenced daily milk yield (P<0.001), protein content (P<0.001 and P<0.01) and milk urea concentration, but only for Holstein breed (P<0.05). Milking time significantly influenced the fat and protein content (P<0.001) in the milk of Holstein cows, that is, lactose content (P<0.05) and urea concentration (P<0.05) in the milk of Simmental cows. The season significantly influenced the fat and protein content of milk (P<0.001), that is, urea concentration and log somatic cell count (P<0.01). Determining of urea concentration in cow milk should also be systematically conducted in the Republic of Croatia, in order to determine standard physiological values characteristical for a particular cow breed, aiming to determine the balance of energy and protein in rations.Određivanje koncentracije ureje u mlijeku koristan je pokazatelj opskrbljenosti organizma proteinima, kao i odnosa energije i proteina u obroku preživača, te ima sve veću praktičnu primjenu. Na
koncentraciju ureje u mlijeku, osim hranidbe utječe niz čimbenika kao što su: pasmina, stadij i redoslijed laktacije, tjelesna masa, dnevna proizvodnja i kemijski sastav mlijeka, broj somatskih stanica, sezona i mužnja. Cilj istraživanja bio je utvrditi utjecaj pasmine krava (holstein i simentalska), redoslijeda laktacije (1., 2., 3., 4. i 5.), mužnje (jutarnja - večernja) i sezone (proljeće-ljeto i jesen-zima) na koncentraciju ureje u mlijeku. Za svaku pasminu utvrđena je dnevna količina mlijeka, udio mliječne masti, proteina, laktoze, koncentracija ureje i broj somatskih stanica u mlijeku. Statistička obrada podataka izvršena je primjenom procedure General Linear Model, programskog sustava SAS (1999). Pasmina krava značajno je utjecala na dnevnu količinu mlijeka i log broj somatskih stanica (P<0,001), udio laktoze u mlijeku (P<0,01), udio mliječne masti i koncentraciju ureje u mlijeku (P<0,05). Redoslijed
laktacije značajno je utjecao na dnevnu količinu mlijeka (P<0,001), udio proteina u mlijeku (P<0,001 i P<0,01) te na koncentraciju ureje u mlijeku ali samo kod holstein pasmine (P<0,05). Vrijeme mužnje
značajno je utjecalo na udio mliječne masti i proteina (P<0,001) u mlijeku holstein krava odnosno na udio laktoze (P<0,05) i koncentraciju ureje (P<0,05) u mlijeku simentalskih krava. Sezona je imala značajan utjecaj na udio mliječne masti i proteina u mlijeku (P<0,001) odnosno na koncentraciju ureje i log broj somatskih stanica (P<0,01). Određivanje koncentracije ureje u kravljem mlijeku trebalo bi sustavno provoditi i u RH, kako bi se utvrdile standardne fiziološke vrijednosti karakteristične za pojedinu pasminu krava, a u cilju procjene izbalansiranosti obroka energijom i proteinima
