This study aimed to estimate milk urea nitrogen (MUN) as a function of days in milk (DIM), lactation number (LN) and calving season. In the study, 7.006 observations were used. Milk production (kg), DIM and LN were collected on the milk sampling days. Calving seasons (CS) were divided into summer and winter. A model to describe the lactation curve was used to estimate MUN, adding LN and CS to it and verifying the coincidence among curves. Concentrations of MUN showed similar pattern to the lactation curve and differed among 1^st, 2^nd, 3^rd and 4^th-5^th lactation, being influenced by CS.