Because lithium salts are widely used for long-term therapy of affective disorders and have been recently implicated as a cause of tubulointerstitial renal disease, we have undertaken experiments designed to establish the site of the early and late pathologic lesions and to determine their correlation with the lithium-induced concentrating defect. Male Wistar rats given a semisynthetic diet that contained lithium carbonate, 90 mEq/kg dry weight, developed serum lithium levels in the human therapeutic range; pair-fed controls received sodium carbonate. Within 3 weeks, treated rats developed marked polyuria, with elevation of free water clearance and vasopressin-resistant diabetes insipidus. Early morphologic changes were confined to the cortical collecting tubules and, possibly, contiguous portions of distal tubules. The tubules were dilated and irregularly lined with cells that had bulging or thinned basophilic cytoplasm, enlarged nuclei, sometimes basal vacuolization, and a few mitoses. These changes were evident at 3 weeks and progressed through the end of the observation period at 18 weeks. The proliferative component of the lesion was demonstrated by the finding of a significant and specific increase in 3H-thymidine uptake by nuclei of collecting/distal tubules of lithium-treated rats. The lesion, but not the increased thymidine uptake, extended into the medullary collecting ducts at 9 and 18 weeks. Although occasional intratubular mononuclear cells were seen at 9 and 18 weeks, no interstitial inflammation or fibrosis was seen. These tubular epithelial lesions were not seen in the kidneys of Brattleboro rats or glucose-treated Wistar rats despite comparable polyuria. We suggest that this early, persistent, and reproducible lesion, characterized by reactive and proliferating tubular cells in the cortical collecting tubules, predisposes the kidney to injury from otherwise mild or insignificant insults and may explain the sporadic occurrence of serious tubulointerstitial disease in patients on long-term lithium therapy.