Mice in mid lactation (n=6 per group) were fed a control diet (1·19% fat), or diets containing safflower oil (25% w/w) or olive oil (25% w/w) for 7 d. Mammary and liver stearoyl CoA desaturase (SCD) mRNA levels and mammary SCD activities were higher in lactating mice fed the control diet than in those fed the oil-supplemented diets. Further, mammary SCD mRNA was directly related to mammary SCD activity. Milk fat composition was influenced by dietary fat composition. The olive oil diet, high in 18[ratio ]1, led to high levels of this fatty acid in milk and the safflower oil diet, high in 18[ratio ]2, resulted in a milk fat with high levels of 18[ratio ]2. These results show that there is regulation of SCD at the transcriptional level, associated with changes in enzyme activity and in milk fat composition.