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The Programme d'Analyse des Troupeaux Laitiers du Québec (PATLQ) has been collecting data in Quebec dairy herds on lactose since 2001 and milk urea nitrogen (MUN) since 1997. While data on MUN is also being collected in other Canadian provinces, testing for lactose concentration in Canada is currently done exclusively in Quebec by PATLQ. Concentrations of MUN are measured at Canadian DHI labs by infrared technology. Several studies have already reported associations of MUN and lactose with fertility, health and/or energy balance traits in dairy cows. The objective of this research was to estimate heritabilities of MUN and lactose in the first three parities, and their genetic relationships with milk, fat, protein and SCS in Canadian Holsteins. Multiple-trait random regression test-day animal model and the Gibbs sampling method were used for parameter estimation. Regression curves were modelled using Legendre polynomials of order four. Data were a random sample of test day records (60,645 records from 5022 cows) extracted from the original data set, which included 1,183,725 records from 235,760 Holstein cows. A total of six 4-trait analyses, which included MUN and/or lactose (yield or percentage) with different combinations of production traits, were performed. One chain of 100,000 cycles was run for each 4-trait analysis and the first 10,000 samples were discarded as a burn-in. Average daily heritabilities were moderately high for MUN (0.384 - 0.414), lactose kg (0.466 - 0.539), and lactose percentage (0.478 - 0.508). Lactose kg was highly correlated with milk yield (0.979). Lactose percentage and MUN were not genetically correlated with milk yield. Research is underway to assess the relationships between MUN and lactose with fertility traits. [ABSTRACT FROM AUTHOR]