In order to determine the effect of salinity on the nodule conductance, oxygen uptake by the nodulated roots was measured by registering the concentration of O(2) as a function of time in a tight incubator of known volume containing the nodulated roots of Medicago truncatula. Four lines, namely TN8.20 and TN6.18, originated from local populations, F83005.5 originated from Var (France) and Jemalong 6, a cultivar from Australia, were hydroponically grown in 250 ml glass bottles under semi-controlled conditions in a glasshouse, after germination and inoculation with the strain Sinorhizobium meliloti 2011. The saline treatment was applied gradually to reach 75 mM after 2 weeks. Results show that oxygen uptake increased significantly with salinity in TN6.18 and F83005.5, but not in Jemalong nor in TN8.20. Without salt, Jemalong showed a significantly higher O(2) uptake of 240 micromol O(2) per h per plant than the mean of 130 micromol O(2) per h per plant for other lines. Salinity increased significantly the nodule conductance in all genotypes. This salt effect was significantly higher for TN6.18 than for TN8.20, and for Jemalong than for F83005.5. Without salt there was less genotypic variation in nodule conductance in the range of 5-8 microm s(-1) for F83005.5 and TN8.20, respectively. Thus the sensitivity to salinity appears to be associated with an increase in nodule conductance that supports the increased respiration of N(2)-fixing nodules under salinity.