Perfluorooctylbromide (PFOB) enhances the echogenicity of perfused tissues on sonography. Since PFOB is not filtered and is limited to the intravascular space, the particles are concentrated in the vasa rectae as they travel across the osmotic gradient. Because sonography has been unable to detect renal function, we aimed to determine whether sonography when aided by PFOB could detect and distinguish the normal from the abnormal osmotic gradient. The sonographer, unaware of rabbit assignment, imaged both kidneys in 17 rabbits before and 24 hours after the temporary occlusion of one of the renal arteries and then again after the infusion of up to 5 ml/kg of PFOB (N = 10) or saline (N = 7). Two normal rabbits were imaged before and after PFOB infusion and then again after i.v. furosemide. Without PFOB, the normal and impaired kidneys were indistinguishable. The echogenicity of the medulla which was darker than cortex in normal kidneys became brighter than cortex after PFOB (increased by 117% +/- 10%; P less than 0.01). PFOB, which was visible in the renal medulla on real-time sonography, produced an echogenic gradient that increased in brightness towards the papillary tip. Because the medulla of kidneys with ATN mildly increased in brightness after PFOB (increased by 40% +/- 7.8%; P less than 0.01), and because the echogenic gradient produced by PFOB was reversed (decreased in brightness towards the papillary tip), ATN kidneys were distinguished from normal kidneys in all 10 rabbits after 2.5 ml/kg PFOB. Medullary echogenicity produced by PFOB in normal kidneys was lost after furosemide.(ABSTRACT TRUNCATED AT 250 WORDS)