Washed suspensions and cell-free extracts of Psudomonas aeruginosa grown on Lemco agar, to which intermediates of the tricarboxylic acid cycle had been added, were tested for their ability to oxidize succinate, fumarate, malate pyruvate acetate, alpha-oxoglutarate and citrate. Whole orgainisms had lag periods of 2-3 hr. before citrate was oxidized rapidly, except when citrate or acetate had been added to the growth medium. There were lag periods of about 10 min. before rapid an linear oxidation of succinate by organism grown on acetate; of fumarate by organisms grown on acetate or pyruvate; and of acetate by organism grown on malate, pyruvate, alpha-oxoglutarate, fumarate or succinate. There were no lags for malate, pyruvate or alpha-oxglutarate by organisms grown on any of the substrates tested. Organism grown on malonate or Lemco agar to which no additions had been made had lags for all the substrates. Glucose-grown organism had lags for all the intermediates except succinate. Only malonate grown organism oxidièd malonate rapidly and linearly, and organisms grown without malonate had lag periods of 2-3 hr. before oxidizing this substrate. Cell-free extracts from organism with lag periods before the oxidation of citrate, acetate, fumarate, malate, succinate and alpha-oxoglutarate were shown to oxidize these substrates without a lag period. Pyruvate and malonate were not oxidized by any of the extracts including those from organisms grown on these substrates by whole organism but was without effect on the oxidation by cell-free extract. The significance of the findings in relation to the hypothesis of specific permeases for the transport of organic molecules into the cell is discussed