Magnesium is the second most abundant divalent ion in the body, but the effects of this cation on atrial natriuretic peptide (ANP) release have not been examined. The present study was conducted to determine the effect of magnesium on ANP secretion. Experiments were conducted in six groups of male Wistar rats. Each group was assigned a diet containing a different amount of magnesium. Plasma magnesium was 0.42 +/- 0.01, 0.63 +/- 0.01, 0.75 +/- 0.02, 0.97 +/- 0.03, 1.03 +/- 0.01, and 1.19 +/- 0.01 mM in groups I, II, III, IV, V, and VI, respectively. Plasma ANP concentration was significantly higher in the hypermagnesemic animals and significantly lower in the hypomagnesemic rats. A significant positive correlation was found between plasma magnesium and plasma ANP levels (y = 88 + 23 chi; r = 0.46; P less than 0.01). ANP concentration in the atria was lower in hypomagnesemic rats and higher in hypermagnesemic rats. This suggests that the low concentrations of ANP found in the plasma of hypomagnesemic animals were due to the lack of ANP in the atria. The atria from the various groups were isolated and perfused in a modified Langendorff apparatus to measure the rate of ANP secretion. Our results showed that the hypomagnesemic rats have a lower release rate as opposed to that seen in hypermagnesemic animals. A significant correlation was also seen between ANP secretion and tissue ANP concentration. The higher rate of ANP release from the heart of hypermagnesemic animals was due to the presence of more ANP, which was reduced during hypomagnesemia.(ABSTRACT TRUNCATED AT 250 WORDS)