1. The depression of p-aminohippurate and o-iodohippurate transport induced by uraemic serum in rabbit and rat renal cortex in vitro was confirmed. At the same time no change in oxygen consumption was observed.
2. The transport of the tetraethylammonium ion was unimpaired by uraemic serum from man or rat.
3. When sera from neomycin-treated uraemic rats were incubated with rabbit renal cortex, the uptakes of p-aminohippurate and of o-iodohippurate were 50% higher than when sera from untreated uraemic rats were used.
4. In rat uraemic serum, the levels of hippuric and indole acetic acids are markedly elevated. Administration of neomycin causes a substantial reduction in these levels.
5. The implications of these observations upon the impaired renal transport of hippurates produced by uraemic serum is discussed.