To the Editor: I read, with some dismay, the article by Kimberly and Plotz1 implying that the rise in serum creatinine produced by high doses of oral salicylates is synonymous with a true depression of glomerular filtration rate. Since creatinine is both filtered and secreted by the renal tubule in man, pharmacologic agents can potentially depress secretion without affecting the true glomerular filtration rate. In the discussion of their paper, the only studies cited that measure inulin or 51chromium-EDTA clearance showed no change during aspirin therapy.23 Recently, trimethoprim–sulfamethoxazole has been reported to affect serum creatinine in a similar manner. © 1977, Massachusetts Medical Society. All rights reserved.