Tissue levels of lactate, pyruvate and the adenine nucleotides were measured in samples of human placenta obtained (1) immediately on delivery, (2) after perfusion on the maternal side for one hour, and (3) after a corresponding period of warm ischaemia. Metabolic activity in the isolated perfused placental lobule was assessed in terms of these tissue metabolites and by measuring protein synthetic rate by means of determining the incorporation of a labelled amino acid. Perfusion was found to lower significantly the lactate level whereas ATP was maintained at comparable levels with those in the placenta just after delivery. Perfused placental ATP levels are lower than in vivo levels seen in other tissues with high metabolic rates such as rat liver, kidney and also in guinea-pig placenta. Protein synthetic rate was found to be lower than that observed in other fetal tissues.