The effect of acute and chronic administration of methylprednisolone on renal function was studied in rats with clearance and micropuncture techniques. Although acute administration did not influence renal function, chronic administration for 4 days caused a marked increase in glomerular filtration rate and PAH clearance. The rise in the reabsurption of sodium under these conditions was associated with a disproportionate increase in filtration rate and renal blood flow in deep portions of the cortex. There was no evidence of altered fractional sodium reabsorption in the proximal segment of surface nephrons by micropuncture analysis or in the ascending limb by clearance studies during a water or a solute diuresis.KEY WORDS methylprednisolone sodium reabsorption micropuncture microspheres glomerular filtration PAH clearance proximal tubules• The effect of glucocorticoid hormones on renal function is of importance because of the striking alterations which occur in adrenal insufficiency and the widespread pharmacological use of these agents in patients with intact adrenal function. Previous studies, in the absence of adrenal insufficiency, have suggested that glucocorticoid hormones increase glomerular filtration rate (1, 2) and free water clearance (3, 4) and may influence the tubular reabsorption of sodium (5, 6).In the present study the action of methylprednisolone on sodium reabsorption by the renal tubule was examined in normal rats under conditions of acute and chronic administration by clearance and micropuncture techniques. In addition, since glucocorticoid From the