In rats, single intravenous doses of folic acid induce damage to renal tubular epithelium, deposition of folic acid in tubular lumens, increase in wet kidney weight, oliguria and interstitial connective tissue proliferation. Separation of the nephrotoxic and obstructive effects of folic acid was attempted by pretreatment with NH4Cl or NaHCO3. These effects of folic acid were unaltered by pretreatment with NH4Cl and there was, in addition, accumulation of eosinophilic droplets in papillary collecting duct epithelium. After pretreatment with NaHCO3, folic acid deposition is decreased or absent; there is a smaller increase in wet kidney weight; the rats are polyuric rather than oliguric; interstitial connective tissue proliferation is reduced; and no droplets form in papillary collecting ducts, but lesions are still present in proximal convoluted tubule epithelial cells. These findings indicate that folic acid has direct nephrotoxic effects independent of intraluminal folic acid deposition, and that damage to renal epithelium, unlike that induced by many nephrotoxins, occurs at several levels of the nephron.