Susceptibility of alfalfa (Medicago salivaL. cv. Aragón) nodules and leaves to water stress has been investigated. Nodule acetylene reduction activity (ARA), leaf CO2exchange rate (CER) as well as soluble protein, proline and total soluble sugar (TSS) contents were determined during drought. Water status was estimated as water potential (Ψw) and Relative water content (RWC) of the respective tissues. Maximum rates of ARA required higher Ψwthan CER. Nodules had lower RWC for a given Ψwthan leaves. Water stress reduced soluble protein content in both tissues; however, the decline in soluble protein content was detected at greater Ψwin nodules than in leaves. Proline and TSS increased in leaves and nodules, and again the threshold Ψwtriggering such accumulation was higher in nodule tissues. Oior results suggest that alfalfa nodules are more susceptible to water shortage than leaves. Effects of accumulated TSS and proline upon leaf and nodule physiology are discussed in relation to protein stability (proline), pH control (proline) and osmotic adjustment (proiine and TSS). The TSS accumulation induced by water stress suggests that substrate shortage would not be the primary effect of drought on nodule act