An intravenous injection of 153Gd-labeled gadolinium-DTPA or gadolinium chloride was given to 60 rats, which were killed either 15, 40, 120, 300, 900, or 3600 seconds later. Tissue concentrations of gadolinium in the blood, liver, spleen, stomach, pancreas, renal cortex, renal medulla, lungs, heart, adrenals, gluteal muscle, fat, skin, thymus, brain, thyroid, and parathyroids were measured. Five animals were killed at each time interval with gadolinium-DTPA, and at each of the latter four time intervals (15 and 40 seconds excluded) with gadolinium chloride. The pharmacokinetics of gadolinium-DTPA and chloride differ markedly in terms of tissue concentration, distribution volumes, and the time course of these parameters. Gadolinium, when injected as a chloride, evidently forms insoluble carbonate and phosphate precipitates in the blood, which are taken up by the reticuloendothelial system. The distribution of gadolinium-DTPA on dynamic MRI should closely parallel the distribution of iodinated contrast media on dynamic computed tomography.