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Magnesium is one of the four principal cations in extracellular fluid, and in intracellular fluid is exceeded only by potassium; yet comparatively little is known about the mechanisms involved in the renal excretion of magnesium. That such is the case must be due to a variety of factors which include difficulties in chemical analysis, the binding by protein of a relatively large portion of the ion in serum, and the formation of soluble complexes which may be handled by the kidney in a different manner from that of the ion. Such difficulties probably account for the relatively few studies to date of magnesium excretion.The experiments reported here involve 70 acute studies in man performed by usual clearance techniques. Renal hemodynamics and excretion rates of magnesium and other electrolytes were observed during multiple control periods and following eight principal experimental procedures. These were intravenous infusions of sodium lactate, sodium bicarbonate, acetazolamide, probenecid, sodium para-aminohippurate (PAH) to tubular saturation, magnesium salts, calcium salts and mercurial diuretics.The data provide a basis for examination of factors likely to affect magnesium excretion and in some instances of probable renal mechanisms involved. The major part of filtered magnesium is reabsorbed by the tubules. The existence of a quantitatively small, but perhaps important, regulatory tubular secretion of magnesium is suggested by certain indirect evidence, but is not settled by present data. In addition to changes in filtered magnesium, consideration is given to possible ef-*Laboratory facilities were aided by grants from the National Heart Institute of the United States Public Health Service fects on tubular magnesium transport of such factors as acid-base balance, possible relation to known active transport systems, specific interionic relationships especially with calcium or potassium, and formation of soluble magnesium complexes. METHODSGeneral. Fifty-three normal men between the ages of 18 and 32 served as experimental subjects for one or more renal clearance studies by standard techniques (4) with bladder catheterization and standard water loading. Control results reported are averages from three or more periods of at least 15 minutes each which followed an equilibration period of at least 45 minutes.Test procedures. The test procedure in each case consisted of the intravenous administration of some electrolyte solution or pharmacological agent. The eight major types of experiments are described below. Additional procedures were carried out to elucidate specific points and will be mentioned in appropriate sections. 1) Sodium lactate, 2.4 mEq. per Kg. body weight, as a hypertonic (0.9 M) solution was given rapidly (10 minutes) in six studies. Whenever results of lactate administration are referred to without qualification in this report it should be understood that these are the studies meant. In four other studies alterations were made in the dosage or time of administration of lactate for comparison wit...

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