Distribution of 4-iodoantipyrine after intravenous injection in the rat.

• The determination of total body water by measurements of the distribution A'olume of antipyrine was originally proposed by Soberman, Brodie, et al. The applicability of the method has been extended by the use of the more easily determined aeetylaminoantipyrine. It has also been suggested' that the iodinated derivative of antipyrine labeled with I might be a satisfactory substitute for the parent compound. In one study, it has been shown that the apparent distribution volumes of antipyrine, acetylaininoantipyrine, and 4-iodoantipyrine are nearly identical in human beings as well as in dogs; and another investigation utilizing sheep gave similar results in the comparison of antipyrine and 4-iodoautipyrine. The ease of determination of the radioactive compound has suggested that this material may be an effective substitute for antipyrine in the determination of total body water. Unfortunately, it has never been shown for iodoantipyrine that it is a true tracer for body water, i.e., that its distribution parallels the distribution of body water. Unless this is shown, as it has been for autipyrine, the validitj' of measurement of body water with iodoantipyrine must remain in question. An indication that the distribution of iodoantipyrine did not parallel the distribution of body water was offered by the work of