Hypophosphatemia as a consequence of potassium deficiency has been reported sporadically. Most cases have been complicated by other factors which might lead to decreased serum phosphorus levels. Therefore, the serum phosphorus in this study was measured in Sprague-Dawley rats with nutritionally induced potassium deficiency. Severe potassium depletion was manifested by hypokalemia (2.4 mEq/liter versus 3.9 mEq/liter in controls) and decreased muscle potassium content. Statistically significant hypophosphatemia did not develop, although decreased muscle phosphorus content was observed. Therefore, hypophosphatemia is not a regular accompaniment of severe potassium deficiency in the rat.