peer reviewed ; The contents of saturated (SAT) and monounsaturated (MONO) fatty acids in milk vary through the lactation. The aim of this research was to estimate the genetic parameters for these groups of fatty acids in bovine milk using multi-trait random regression test-day models. The relationship between these fatty acids and the milk production traits was also studied as well as the effects of season and stage of lactation. The model included as fixed effects: herd x date of test, class of 15 days in milk, class of age. Random effects were herd x year of calving, permanent environmental, additive genetic, and residual effects. Since October 2007, all spectra generated by milk recording in the Walloon part of Belgium were collected. The contents of SAT and MONO were estimated by mid-infrared spectrometry. First results were obtained from 100,841 test-day records from 11,626 Holstein heifers. The season and stage of lactation influenced the contents of SAT and MONO in milk and milk fat. The heritability estimates of SAT and MONO in fat were higher at the beginning of the lactation. The negative energy balance of cows could explain this variation. The values of heritability were also higher at the end of lactation. The genetic and phenotypic correlations among studied traits varied also through the lactation. This study confirms the genetic variability of fatty acids and suggests a possible relation between the contents of SAT and MONO in milk fat and the energy balance of cows