Using 31P magnetic resonance spectroscopy, localized spectroscopy and magnetic resonance imaging we studied effects of acute urinary obstruction in the in vivo pig kidney. Accumulation of urine in the renal pelvis and collecting ducts resulted in the appearance of a new peak in the localized phosphorus spectra originating in the renal papilla, resonating at 3.43 to 4.56 ppm. This was inorganic phosphate with a pH of 5.60 to 6.79 (urine pH). Imaging did not show any dilatation of renal pelvis. There was a significant time dependent fall in renal [ATP] during urinary obstruction followed by a rapid "overshoot" of [ATP] and disappearance of the phosphate peak after release of obstruction. Possible mechanisms for this phenomenon are discussed. We conclude that 31P magnetic resonance spectroscopy provides early evidence of urinary obstruction in vivo and could be of value in clinical diagnosis.