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To elucidate the morphological basis of renal lesions due to intravenous administration of radiocontrast media, a comparative study was performed by injecting contrast media with different properties, such as high-osmolar ionic diatrizoate (60% Urografin), low-osmolar ionic ioxaglate (Hexabix 320) and low-osmolar non-ionic iopamidol (lopamiron 300), via the tail vein of male Wistar rats. Physiologic saline was injected in controls. Each contrast media was administered singly as a low dose (1 g I/kg), a moderate dose (3 g I/kg) or a high dose (9 g I/kg) to three separate groups of rats. In additional groups of rats, a high dose of each contrast media (9 mg I/kg) was administered repeatedly two or four times. The animals were sacrificed chronologically from 5 minutes to 30 days after the last administration and their renal tissues were examined by light and electron microscopy. The results indicated that the low osmolar contrast media is beneficial in decreasing general symptoms, but there are no definite differences between high and low osmolar contrast media in the data of blood and urine examination after administration. The iodinated contrast media with a low or moderate dose does not induce definite histopathological changes of the renal tissue, however, with a high dose it causes vacuolization of proximal tubular epithelia, although this change recovers within three days. The repeated administration of contrast media with a high dose induces severe damage of proximal tubular epithelia with prominent vacuolization, appearance of intracytoplasmic granular structure and occasional cell necroses. The vacuolization appears even in distal and collective tubules and glomerular epithelia. Recovery is retarded under this condition. There are no qualitative histological differences of renal lesions induced by the three different contrast media. In conclusion, either high or low osmolar contrast media induces almost the same histological alteration in renal tissues under the same experimental conditions. The more the quantitative increase in administered iodinated contrast media, the more prominent is the histological impairment of the kidney. The degree of renal lesion and its recovery might depend on the dose of iodine rather than the osmolality of the contrast media.