According to the fetal programming hypothesis, impaired intrauterine development results in insulin resistance and associated metabolic disturbances. Recently, we reported increased body fat, a forerunner of insulin resistance, in the pups of mineral-restricted rat dams. To identify the causative mineral(s), the effect of magnesium restriction was assessed. Female weanling WNIN rats (n = 21) consumed ad libitum for 9 wk a 70% magnesium-restricted diet or were pair-fed a control (C) diet (n = 7). After 9 wk, they were mated with control males. Control dams and pups were fed the control diet throughout, whereas 7 Mg-restricted dams were switched to the control diet at parturition and their pups weaned onto the control diet (RP). Pups of the remaining 14 restricted dams were weaned onto the control diet (RW) or the Mg-restricted diet (R). All groups had 8 male pups from weaning. Pups were studied on postnatal d 90 and 180. R pups weighed less than C pups at weaning, but both RP and RW pups caught up with controls by d 90. At this time, R pups were neither insulin resistant nor glucose intolerant, but had a higher percentage of body fat and plasma triglycerides and lower lean body and fat-free mass than C pups. These variables were partially corrected in both RP and RW pups. On postnatal d 180, R, RP, and RW pups were insulin resistant and had a lower insulin response to a glucose challenge than C pups; however, glucose tolerance was impaired only in RW pups. Thus, maternal magnesium restriction irreversibly increases body fat and induces insulin resistance in pups by 6 mo of age, whereas additional perinatal Mg deficiency impairs glucose tolerance.