An important characteristic of hyperlipaemia associated with magnesium deficiency in rats is the postprandial accumulation of triglyceride-rich lipoproteins (TGRLP). The present investigation was performed to determine the susceptibility of TGRLP isolated from magnesium-deficient rats to metal ion and cell dependent peroxidation and the effect of these lipoproteins on cultured vascular smooth muscle cells (VSMC). TGRLP were isolated by sequential ultracentrifugation from plasma of control and magnesium-deficient rats fed for 8 d on adequate or magnesium-deficient diets. Magnesium-deficient animals were hypertriglyceridemic as compared to control rats. After exposure to oxidative stress in vitro, Cu2+ or activated macrophages, conjugated diene production was significantly higher in TGRLP isolated from magnesium-deficient rats. Culture of VSMC with TGRLP from magnesium-deficient rats resulted in more important cell growth than with lipoproteins isolated from control animals. After incubation with VSMC, TGRLP from magnesium-deficient rats showed higher conjugated diene production than those from control rats. These results support the atherogenic properties of magnesium deficiency, which is accompanied by hyperlipaemia and which affects two important linked pathways in atherosclerosis, lipoprotein peroxidation and VSMC proliferation.