Chromatographic analysis of BSP (sodium phenoltetrabromphthalein disulfonate) excreted into the bile of the rat showed that the BSP had undergone transformation into similarly colored but chemically different compounds. Injection of purified samples of these derived compounds into the rat showed that the compounds were less effectively removed from plasma than BSP but were excreted into bile at almost the same rate as when pure BSP was injected. Chromatographic analysis of bile following injection of the isolated bile-type derivatives of BSP showed that the derivatives were formed, one from the other, in a chain sequence, starting with BSP. Chromatographic analysis of the blood following injection of BSP showed that bile-type derivatives of BSP appeared in the blood and became progressively more predominant with increasing time after injection of the BSP.