Fifteen healthy volunteers were put on a mackerel and herring diet, consisting of a prescribed daily isocaloric regimen in a cross-over design, for 2 weeks. Eicosapentaenoic acid (EPA - C20:5, n-3) was predominantly incorporated into cholesterol esters, whereas docosahexaenoic acid (C22:6, n-3) appeared more in serum triglycerides, indicating that the function of the latter may be different from that of EPA. After mackerel ingestion, serum triglycerides, total cholesterol and lecithin cholesterol acyl transferase (LCAT) activity were significantly decreased, returning to basal levels 3 months later. Low density lipoprotein (LDL) cholesterol, high density lipoprotein (HDL) cholesterol and postheparin lipolytic activity (PHLA) remained unchanged at the end of the mackerel diet. Generally, after the herring diet the differences were minor, only LCAT activity being significantly decreased. A markedly lower systolic and diastolic blood pressure at the end of the mackerel period could be observed. After herring diet a slight diminution of blood pressure was not significant. Accordingly, plasma noradrenaline was only significantly decreased at the end of the mackerel period. Dopamine-beta-hydroxylase (DBH) activity in serum had no differences before, during and after the study. From the data presented it can be said that a mackerel diet exerts a beneficial influence on cardiovascular risk.