In the immature rabbit, a real-time method for monitoring filtration fraction (FF) to demonstrate acute changes in the kidney's response to intravenous injections of contrast media was used. The renal arteriovenous difference of an agent cleared by the kidney as a pure glomerular filtrate was measured with a gamma detector, and FF was calculated by a computer and displayed on a TV monitor. The effect of FF, arterial pressure (BP), and renal blood flow (RBF) of Renografin-60 and an agent with a much lower osmolality, Iopamidol 300, was examined in anesthetized four-to five-week-old rabbits. Renografin (2 and 4 mL/kg, IV) induced dose-related decreases in FF of 40% and 73%, decreased arterial pressure by 12% and 30%, and at the 4 mL/kg dose increased RBF by 77%. Iopamidol (4 mL/kg) induced a smaller, 41% decrease in FF than the same dose of Renografin (P less than .01), a smaller, 28% increase in RBF (P less than .001), and, unlike Renografin, induced a small increase in BP. Comparison of the magnitude of the reduction of FF induced by a 2-mL/kg IV dose of Renografin in the present immature rabbits with previous results in mature rabbits shows a surprising similarity (P = .33), despite lower resting BP, RBF, and glomerular filtration rate, as well as reduced tubular function in the immature animals.