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The effects of dietary calcium and phosphorus levels on audiogenic seizure susceptibility and brain neurotransmitter were investigated in magnesium deficient rats. For 17 days, male weanling rats were fed magnesium deficient diets which also contained deficient (-), adequate (=}, and excess (+) amounts of calcium and/or phosphorus. Reduction of seizure incidence was seen in low calcium and/or low phosphorus diets. High calcium, and high calcium in combination with high phosphorus increased the severity of seizures. High calcium and low phosphorus, and high phosphorus and low calcium diets prevented seizuring. Most magnesium deficient diets resulted in elevation of serum phosphorus, calcium, and sodium concentrations. Within diet treatments, animals that seized had higher serum mineral concentrations than animals that failed to seize. Magnesium deficiency increased serotonin in cerebral cortex, cerebellum, and medulla oblongata and pons, and 5-hydroxyindoleacetic acid concentrations in cerebral cortex. There were no significant diet effects in brain neurotransmitter concentrations in the midbrain. Calcium seemed to play an important role in increasing both audiogenic seizure susceptibility and brain neurotransmitters. However, brain neurotransmitter levels were not related to seizure susceptibility. Calcium increased serum potassium and sodium concentrations and it increased brain serotonin concentration overall. Increasing dietary phosphorus levels increased serum phosphorus decreased serum calcium concentrations, but did not affect brain neurotransmitters.