The effects of dietary magnesium (Mg) deficiency on histamine metabolism were studied. Young Wistar rats were fed a Mg-deficient diet (0.001% Mg diet) ad libitum for 8 days with control groups (0.07% Mg diet), food-restricted groups (0.21% Mg diet, but restricted to 5 g/rat/day), and refeeding groups (0.001% Mg diet for 6 days ad libitum, after that fed with a 0.21% Mg diet ad libitum for 2 days). Compared to the other groups, the plasma Mg level was markedly lower in the Mg-deficient group. A return from the lower Mg level to the controls took place after feeding them a 0.21% Mg diet for 2 days. Urinary histamine level increased rapidly after 4 days and reached a maximum on the eighth day of Mg deficiency. The high urinary histamine level in Mg-deficient rats decreased rapidly after feeding them a 0.21% Mg diet for 2 days. Histamine contents in some tissues increased on the eighth day of Mg deficiency. Other groups showed no significant change. The increased histamine content in Mg-deficient rats showed a tendency to return to control levels after feeding them a 0.21% Mg diet for 2 days. Histidine decarboxylase (HDC) activity in some tissues of Mg-deficient rats increased markedly. The increased HDC activity dropped nearly to control levels after feeding them a 0.21% Mg diet for 2 days. Diamine oxidase (DAO) activity in the duodenum was high in control rats. Duodenal DAO activity decreased gradually and reached half the value of controls on the eighth day of Mg deficiency.(ABSTRACT TRUNCATED AT 250 WORDS)