1. The excretory patterns of sodium, calcium, magnesium and phosphorus were evaluated in seventy-eight patients with varying degrees of renal insufficiency, and in twelve normal subjects. 2. In mild renal failure the fractional excretion of filtered sodium, magnesium and phosphorus are significantly higher while that of calcium is significantly lower than values seen in normals. In advanced renal failure the fractional excretions of the filtered loads of all these ions increase steeply. These patterns are not influenced by the type of renal disease. 3. The fractional excretions of filtered sodium and calcium, calcium and magnesium, and sodium and magnesium were plotted against each other. The analysis of the plots showed that the fractional excretions of their filtered loads correlate poorly in patients with mild and moderate renal insufficiency, but closely in patients with severe renal failure. 4. These results indicate that the relation between the renal handling of divalent ions is not uniform at all levels of renal insufficiency. The dissociation between the excretory patterns observed in patients with early renal failure is consistent with the concept that different mechanisms may influence each individual ion separately. This is in contrast to the close association which exists between the excretory patterns of these ions in patients with advanced renal disease, and in which a single common mechanism may underlie the renal handling of these ions.