summary ¬_Arginine transport by the fetal side of human placenta was investigated through the characterization of ¬_ [ÅH]arginine uptake in isolated perfused cotyledon. Competitive inhibition experiments suggest the presence of at least two transport systems: a Na¤-independent, pHinsensitive system inhibitable by cationic amino acids, similar to system y¤, and a Na¤-dependent system which recognizes both cationic and neutral amino acids only in the presence of Na¤, i.e. a B°, + -like system. The kinetic analysis of ¬-arginine uptake in the presence of Na¤ revealed that the process is mediated by saturable components: a high-affinity system (Km = 167 ± 18·0 ìÒ; Vmax = 0·174 ± 0·012 ìmol min¢) and a low-affinity carrier (Km = 980 ± 112 ìÒ; Vmax = 1·60 ± 0·12 ìmol min¢). In the absence of Na¤, ¬_arginine uptake was fitted by one model with a Michaelis-Menten constant of 200 ± 24·8 ìÒ. These results suggest that the high-affinity component corresponds to the Na¤-independent system y¤, whilst the low-affinity system may represent the activity of the Na¤-dependent B°, + transporter. Kinetic studies in placentae taken from aspirin-treated pregnancies showed that ¬_arginine is transported with a significantly higher affinity (Km = 42·5 ± 5·7 ìÒ), but with a lower capacity (Vmax = 0·064 ± 0·003 ìmol min¢) than in the non-treated group. The latter finding suggests that aspirin would facilitate the uptake of the NO precursor only at very low arginine concentrations.