The authors tested a noninvasive technique for magnetic resonance imaging of perfusion in human kidneys. Magnetic labeling (spin tagging) of aortic arterial water was performed to generate an endogenous tracer. Breath-hold renal perfusion images obtained in seven volunteers yielded average perfusion rates in cortex and medulla of 278 mL.100 g-1.min-1 +/- 55 (standard error) and 55 mL.100 g-1.min-1 +/- 25, respectively.