Nodulated alfalfa (Medicago sativa L. cv. Saranac) plants were grown in hydroponics at P-sufficient and P-deficient supply levels. After 5 weeks of growth, dry matter accumulation, nodulation, total N and P accumulation, as well as 15N2 uptake, were measured. Moreover, the response of nodule O2-uptake to raising external pO2 was determined in an open-flow measurement system and nodule permeability was calculated. Plants in the P-deficient supply treatment had a lower P concentration in all organs. In both treatments the highest P concentration was found in nodules. In the P-deficient supply treatment plants formed less dry matter, had a lower shoot/root ratio, less nodulation, decreased total N accumulation, and lower 15N2 uptake per dry matter nodule. Nodules in the P-deficient treatment were, on average, smaller and had a higher O2 uptake per N2 reduced, coinciding with increased nodule permeability and conductance. Thus increased oxygen uptake appears to be a mechanism to adjust nodule metabolism to P deficiency in indeterminate N2-fixing nodules such as in alfalfa, as has previously been shown for determinate nodule forms.