We studied renal function, urinary enzymes, urinary sediment, and renal histopathology in Fischer 344 rats that were treated with one dose of mercuric chloride (HgCl2) alone, HgCl2followed by gentamicin, gentamicin alone, or gentamicin, followed by HgCl2. HgCl2was administered intraperitoneally at 1 mg/kg body weight. Gentamicin was injected subcutaneously at 40 mg/kg body weight. Renal function was assessed by creatinine clearance. Urinary sediment was examined using transmission electron microscopy; particular attention was given to the numbers by myeloid bodies in the urinary sediment. Renal tissue was assessed using light microscopy for acute tubular necrosis (ATN). In either HgCl2- or saline-treated rats urinary sediment showed no myeloid bodies, and renal morphology was essentially normal. The rats given HgCl248 h prior to initiation of gentamicin therapy showed significant decrease of myeloid bodies excretion. This was accompanied by significantly less impairment of renal function, mild renal lesion, and no necrotic tubule cells in urinary sediment. The rats treated with either gentamicin alone or gentamicin followed by HgCl2developed significant impairment of renal function in association with marked elevation of the urinary enzymes, and variable extent of ATN. In both of these groups, urinary sediment showed a profusion of free myeloid bodies and many necrotic renal tubule cells. The urinary sediment findings, however, did not aid in distinguishing between these two treatment groups. From these data we conclude that (1) a tentative relationship exists betweeen the concentration of the urinary myeloid bodies and severity of gentamicin nephrotoxicity; (2) prior treatment with compound(s) analogous to HgCl2which could minimize urinary excretion of the myeloid bodies might be useful in the mitigation of gentamicin nephrotoxicity.