Preeclampsia is a hypertensive disorder that is unique to pregnancy. Magnesium (Mg2+) supplementation is a potential new therapy to ameliorate development of hypertension. The aim of this work was to compare the effects of Mg2+ supplementation on systolic blood pressure (SBP) in young and aged rats. Spontaneously hypertensive rats (SHR) were divided into young (6-week-old male, n = 10) and old (16-week-old male, n = 10) groups. Each group of rats comprised two subgroups made of a control subgroup fed with normal rat chow (0.2% Mg2+, n = 5) and a high Mg2+ subgroup nourished with a Mg2+ rich diet (0.8% Mg2+, n = 5). Age-matched Wistar-Kyoto rats (WKY) were also allocated into two groups. SBP was assessed weekly for 12 weeks indirectly by the tail-cuff method. SBP increased progressively in SHR-young rats after 7 weeks. This increase was greater in the control subgroup compared to high Mg2+ subgroup at 7 weeks (p < 0.05). No difference in the SBP was registered between old SHR subgroups. Mg2+ supplementation does not exert antihypertensive effects in the WKY rats. In conclusion, Mg2+ supplementation may provide beneficial effect in the developmental phase of hypertension but not in established hypertension.