1. The patterns of the free plasma amino acids in the pregnant guinea-pig and her foetuses, near term, are described. The concentration of each amino acid was higher in the foetal plasma than in the maternal. The foetal:maternal gradients (F:M) varied for each amino acid; the straight chain amino acids had the highest F:M ratios.2. Net transfer of endogenous plasma amino acids, from the maternal circulation across the placental membrane, was studied. The foetus was removed and the foetal placenta perfused in situ via the umbilical arteries, with an artificial fluid containing varying concentrations of amino acids.3. All the amino acids, both essential and non-essential, could be transferred from the maternal to the foetal circulation against the F:M gradients. With ;closed circuit' perfusion, this transport increased the concentration of total amino N in the perfusate until it was twice that of the normal F:M gradient of 5. The concentrations of the individual amino acids was increased to 1.7-4.2 times those normally present in foetal plasma, and the final values reached were similar to the concentrations of free amino acid found in placental tissue.4. The umbilical vein-artery differences were small, with the placenta perfused ;open circuit' in the steady state, using physiological flow rates and amino acid concentrations. The average net placental transfer of amino N found was 1.14 m-mole min(-1). This is about 60% of the calculated net rate of accumulation of N by the 60 g guinea-pig foetus.5. The influence of foetal placental perfusion concentration on transfer was small but significant. In the steady state, the transfer of amino N, and each individual amino acid, was found to be inversely proportional to the concentrations in the perfusate when the placenta was perfused ;open circuit'. The slopes of the regression of transfer on concentration had an average value of 0.13 n-mole min(-1) g(-1) per mumole. No significant difference in the slopes was found between the three amino acid transport groups.6. Net transfer was independent of perfusate flow, within the physiological range, which suggests a secretory process across the membrane from maternal to foetal circulation.