Kidneys of six wild-trapped, adult Norway rats with high agglutinating titers to Leptospira icterohaemmorhagiae were examined ultrastructurally. Although many leptospires were seen in the lumen of proximal kidney tubules, few changes in epithelial cells of this region were found. The predominant lesion in chronic rat leptospirosis was diffuse interstitial nephritis. Principal kidney lesions were proximal tubule cell basement membrane thickening with or without the presence of granular particles, and an interstitial response that varied from mixed hypercellular to chiefly plasmacytic. The association of leptospires with membrane-bound protein droplets of the proximal kidney tubules may be important in the maintenance of the chronic carrier state in rat leptospirosis.