The effects of moderate magnesium deficiency on the outcome of pregnancy, the process of parturition and uterine involution, were studied in successive experiments using Sherman rats. Females were fed purified diets containing 0.110 or 1.500 g Mg/kg throughout pregnancy and lactation. This magnesium deficient diet decreased the plasma magnesium but did not affect the number of pregnant rats or the weight of females at parturition. There was no change in the weight and size of litter. The number of still births was not significantly increased. The parturition process was observed; abdominal contractions were less obvious in deficient animals than in controls, and they were often not noticeable. Parturition was slightly lengthened in deficient rats. The major effects of magnesium deficiency occurred during lactation when the dam lost weight and the growth of her young was reduced. The uterine weight and its collagen content was higher and the collagen solubility was lower in the deficient rats than in controls between 2 and 10 days post partum. The delay in uterine involution could be prevented by provision of a magnesium supplement after parturition.