Magnesium deficiency in rats is known to precipitate audiogenic seizures. An unknown mineral factor in a diet mixture was found to substantially reduce the seizure occurence in magnesium deficient vii animals. This was corrected when the same mineral mixture was remade. Subsequently the faulty mineral mixture was discarded. This research .. as aimed at determining the mineral factor responsible for the observed changes in seizure occurence, and also aimed at investigating the relationships of serum calcium, magnesium and phosphorus concentrations to seizure susceptibility. Treatments to change the serum concentrations of these minerals included dietary manipulation, subcutaneous injection of calcitonin, of 1 ,25-dihydroxycholecalciferol, of calcium and of phosphorus. Animals fed magnesium deficient, low phosphorus diet and magnesium deficient animals fasted over-night were found to have significantly lower audiogenic seizure susceptibility than all other magnesium deficient animals regardless of treatment. Reduced audiogenic seizure susceptibility was not produced by any of the injections. The reduction in seizure with the magnesium deficient, low phosphorus diet indicates that phosphorus is an important factor in the mechanism of audiogenic seizuring in magnesium deficient rats.