The distribution of creatinine, one of the toxic guanidine compounds, in various tissues has not been studied in detail by using radiolabeled creatinine. Our objective was to investigate the biodistribution of creatinine labeled with (99m)technetium (Tc-99m) by the stannous (II) chloride method in healthy male Wistar rats. Quality controls were carried out by radio thin layer chromatography, high- performance liquid chromatography, and paper electrophoresis. The labeling yield was 85 +/- 2% under optimum conditions (pH 7 and 100 mu g stannous chloride). Rats (N = 12) were injected intravenously with Tc-99m-creatinine and their blood and visceral organs were evaluated for Tc-99m-creatinine uptake as percent of the injected dose per gram wet weight of each tissue (% ID/g). The lowest amount of uptake was detected in the brain and testis. When the rate of uptake was evaluated, only the kidney showed increasing rates of uptake of Tc-99m-creatinine throughout the study. Kidneys showed the highest amount of uptake throughout the study (P < 0.001 compared to all other organs), followed by liver, spleen and lung tissue.