Contrast media affect renal hemodynamics. Hyperosmolality is regarded as the major factor responsible for renal hemodynamic changes. In this study, the role of osmolality was evaluated in 30 hospitalized patients without risk factors during intravenous pyelography. Contrast media with low and high osmolality were used. In addition, nifedipine was administered before infusion of high-osmolality contrast to evaluate the role of calcium ions in radiocontrast-induced changes of renal hemodynamics. Hyperosmolar contrast reduced renal plasma flow and glomerular filtration rate. Calcium channel blocker prevented changes of renal hemodynamics. Hyperosmolality appears the most likely factor affecting renal hemodynamics during hyperosmolar radiocontrast infusion. Calcium channel blocker may prevent renal changes due to hyperosmolar medium.