The objective of this study was to estimate lactation curves of milk urea concentration for primiparous Polish Holstein-Friesian cows, milked in different months and air temperatures, using random regression test-day models. In the study, 6 086 431 test-day records collected in the years 2001 to 2009 from 900 962 cows were analysed. Regression curves in classes of environmental effects were modeled with Legendre polynomials. The pattern of lactation curves for milk urea (MU) resembled the lactation curve for milk. In summer and early fall, the MU concentration was higher than in the winter months. At the same time, the increase in the MU concentration was even more strongly associated with the increase of temperature, also in the winter months. In herds with higher milk production, the MU concentration was also higher, however, interaction between average herd milk production and month-temperature was found. A greater influence of month and air temperature was observed in herds with low milk production.