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Spatial variation in mean annual precipitation is the principal driver of plant water and nitrogen status in drylands. The natural abundance of carbon stable isotopes (delta C-13) in photosynthetic tissues of C3 plants is an indicator of time-integrated behaviour of stomatal conductance; while that of nitrogen stable isotopes (delta N-15) is an indicator of the main source of plant N (soil N vs. atmospheric N-2). Previous studies in drylands have documented that plant delta C-13 and delta N-15 values increase with decreasing mean annual precipitation due to reductions in stomatal conductance, and soil enriched in N-15, respectively. However, evidence for this comes from studies focused on stable isotopes measurements integrated at the plant community level or on dominant plants at the site level, but little effort has been made to study C and N isotope variations within a species growing along rainfall gradients. We analysed plant delta C-13, delta N-15 and C/N values of three woody species having different phenological leaf traits (deciduous, perennial and aphyllous) along a regional mean annual precipitation gradient from the central-western Argentinian drylands. Noticeably, plant delta C-13 and delta N-15 values in the three woody species did not increase towards sites with low precipitation or at the start of the growing season (drier period), as we expected. These results suggest that environmental factors other than mean annual precipitation may be affecting plant delta C-13 and delta N-15. The short-term environmental conditions may interact with species-specific plant traits related to water and nitrogen use strategies and override the predictive influence of the mean annual precipitation on plant delta C-13 and delta N-15 widely reported in drylands.