This study will attempt to develop a simple and accurate method of assessing true glomerular filtration rate (GFR) by comparing the cimetidine-creatinine clearance test (3CT) to the GFR as determined by insulin clearance. The effect of cimetidine on serum creatinine levels and calculated creatinine clearance will be measured. Serum and urine creatinine levels, and calculated creatinine clearance in patients taking cimetidine (3CT) will be compared to the true GFR. A nomogram for relating 3CT to true GF will be constructed.