The effect of radiographic contrast medium on the urine specific gravity of ten clinically normal dogs was evaluated. Urinary excretion of different dosages of intravenously administered triiodinated contrast medium caused significant alterations of urine specific gravity. The magnitude of change was related primarily to preinjection urine specific gravity values rather than dosage of contrast medium. In general, preinjection urine specific gravities < 1.040 were increased 15 minutes after injection of contrast material, whereas preinjection urine specific gravities > 1.040 were decreased 15 minutes after injection of contrast medium. A linear relationship was identified between the concentration of radiographic contrast medium in either distilled water or urine and the specific gravity of these mixtures. This simulated the effects of radiologic contrast medium on urine specific gravity when used for retrograde urethrocystography. Results of the investigation indicated that urinary excretion of radiographic contrast media significantly alters urine specific gravity values in normal dogs and renders postinjection urine samples unsuitable for diagnostic evaluation of renal tubular concentrating capacity. In addition, previously reported errors in urine protein, glucose, and sediment evaluations attributed to radiographic contrast media were reviewed.