Colistin is an old antibiotic used in the treatment of Gram-negative infections. It was once suspended because of its nephrotoxic effect but has since been reintroduced due to multidrug-resistant bacterial infections. The pathogenesis of colistin-associated nephropathy has not been clarified, and there is currently no effective therapeutic or prophylactic agent available. The aim of this study was to investigate the roles of caspase-associated apoptosis and caspase 1, calpain 1, inducible nitric oxide synthase (iNOS), and endothelial nitric oxide synthase (eNOS) expression in the pathogenesis of colistin-associated nephrotoxicity and the effect of grape seed proanthocyanidin extract (GSPE) in preventing it. Twenty-four rats were divided into three groups: control, colistin, and colistin plus GSPE (colistin؉GSPE). Colistin-associated nephropathy was induced by the administration of 300,000 IU/kg of body weight/day colistin intraperitoneally for 7 days. The experiment was discontinued on the seventh day. Blood was collected for measurements of blood urea nitrogen (BUN) and creatinine levels. Histopathological examination of kidney tissue and caspase 1 and 3, iNOS, eNOS, terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL), and calpain 1 staining was also performed. Significant increases in BUN levels; creatinine levels; renal histopathological scores; and TUNEL, caspase 1 and 3, calpain 1, iNOS, and eNOS staining were observed for the colistin group compared to the control group. Significant decreases in BUN levels; creatinine levels; renal histopathological scores; and TUNEL, caspase 1 and 3, calpain 1, iNOS, and eNOS staining were observed in the colistin؉GSPE group compared to the colistin group. Our study shows, for the first time in the literature, that caspase-mediated apoptosis, iNOS, caspase 1, and calpain 1 are involved in the pathogenesis of colistin-associated nephropathy. GSPE had a renoprotective effect, as shown by the lowered levels of these mediators.