Acute renal failure (ARF) is characterized by a very high mortality essentially unchanged over the past 40 years. Simple vertebrate models are needed to improve our understanding of ARF and facilitate the development of novel therapies for this clinical syndrome. Here, we demonstrate that gentamicin, a commonly used nephrotoxic antibiotic, causes larval zebrafish to develop ARF characterized by histological and functional changes that mirror aminoglycoside toxicity in higher vertebrates and inability of zebrafish to maintain fluid homeostasis. We developed a novel method to quantitate renal function in larval zebrafish and demonstrate a decline in glomerular filtration rate after gentamicin exposure. The antineoplastic drug cisplatin, whose use in humans is limited by kidney toxicity, also causes typical histological changes and a decline in renal function in larval zebrafish. A specific inhibitor of Omi/HtrA2, a serine protease implicated in cisplatin-induced apoptosis, prevented renal failure and increased survival. This protective effect was confirmed in a mouse model of cisplatin-induced nephrotoxicity. Therefore, zebrafish provides a unique model system, amenable to genetic manipulation and drug screening, to explore the pathophysiology of ARF and establish novel therapies with potential use in mammals.