We investigated the effect of moderate dietary magnesium (Mg) deficiency, pregnancy, and concurrent L-arginine ingestion on systolic blood pressure (BP), nitric oxide production, serum Mg, and intrauterine growth restriction. Female rats, fed Mg deficient or sufficient diets from weaning, were divided into three groups: (1) nonpregnant, (2) pregnant, and (3) pregnant + 1% L-arginine in the drinking water. The systolic BP, plasma nitrite, serum Mg, and pup weights were determined. Data were analyzed using general linear model analysis of variance. In the pregnant rats, the Mg-deficient group had a significantly higher systolic BP (P =.0275) and plasma nitrite (P =.0244) compared to the Mg-sufficient group. L-Arginine ingestion significantly lowered systolic BP (P <.0001), and increased pup weight (P <.0067). Among the deficient groups, serum Mg was significantly lower (P =.0010) in pregnant rats without L-arginine, but was similar to the nonpregnant state in pregnant Mg-deficient after ingesting L-arginine. Moderate Mg deficiency, during the stress of pregnancy, adversely affected BP, nitric oxide production, and serum Mg. Supplemental L-arginine prevented these negative effects, possibly through a feedback loop by increasing nitric oxide syntheses, which increases cyclic guanosine monophosphate, which then increases free Mg and decreases free calcium. These ionic changes can act to decrease BP and modulate nitric oxide synthesis.