Dietary supplementation with fish oil induces beneficial changes in the size and concentration of plasma lipoproteins, although the underlying mechanism is unclear. We have investigated the effect of increasing the amount of fish oil in a single meal on the size and concentration of VLDL, LDL and HDL particles during the postprandial period. Healthy men aged 58 (sd 5) years (n 11) consumed isoenergetic, mixed macronutrient test meals containing either 0·3 g (reference, REF) or 2·2 g (high fish oil, HFO) fish oil in a randomised order, and blood samples were collected over the following 6 h. Plasma lipoprotein size and concentration were measured by NMR spectroscopy. There was a significant interaction effect of time and meal composition on the VLDL, but not on the LDL or HDL, concentration (P = 0·036) and particle size (P = 0·005). Consuming the HFO meal significantly increased the VLDL concentration (P < 0·05) and reduced VLDL particle size (P < 0·05) when compared with the REF meal and baseline. LDL particle size decreased slightly during the postprandial period, but there was no difference between the meals. There was no effect of time or meal composition in the LDL concentration. The HDL concentration decreased and size increased slightly during the postprandial period, but there were no significant differences between the meals. Increased consumption of fish oil induces acute changes in the VLDL, but not in the LDL or HDL, metabolism.