The effects of water stress on the development of Medicago arborea (MA), Medicago citrina (MC) and Medicago strasseri (MS) plants right after germination were studied in the experimental plot of the UIB. Plants were grown in pots from April until June under irrigation (at field capacity) and under soil water deficit (50 per cent of field capacity). Under irrigation, MS showed higher biomass production and leaf area, followed by MA and MC. Soil water deficit resulted in an important reduction of the parameters studied (around 50 per cent), the same ordering of species being maintained. Under irrigation, water use efficiency was greater in MC and MS, but under water deficit conditions, MC showed higher values. The two endemic species (MC and MS) had higher root/aboveground biomass ratio than MA.