Omega‐3 polyunsaturated fatty acids (PUFA), notably DHA and EPA, impact many metabolic pathways including down regulation of hepatic lipogenesis. Interestingly, fish oil supplements tend to increase mammary de novo synthesized medium chain fatty acids (MCFA) in lactating rodents, suggesting that the effect of PUFA on lipogenesis may be tissue specific. Our objective was to examine the effect of omega‐3 PUFA on mammary lipogenesis. Female C57BL/6J mice were fed a semi‐purified low fat control diet beginning 1 wk prepartum. On d 6 of lactation, mice were maintained on the low fat control diet ( 5% dietary oil) or shifted to high fat diets (10% dietary oil) enriched in either omega‐3 PUFA (fish oil) or oleic acid (control high fat) for 7 d (n = 8 mice/group). Results showed that mammary lipogenic capacity, measured by C14‐glucose incorporation into lipids by mammary explants, was similar among treatments. Moreover, there was no treatment effect on the proportion of MCFA in milk fat (g/100 g fatty acids) or litter weight gain, a proxy for milk energy secretion. In contrast, the omega‐3 rich diet reduced liver weight compared to controls (P<0.001). Overall, dietary omega‐3 PUFA had no apparent effect on mammary lipogenesis at the experimental dietary level. Moreover, when contrasted with the concomitant reduction in liver weight, results suggest that murine mammary lipogenesis may be less‐ or non‐responsive to omega‐3 PUFA.