We examined relationships between stable isotopes of carbon (delta(13)C) and oxygen (delta(18)O) in tree rings of Callitris columellaris F. Muell in the semi-arid Pilbara region of north-western Australia. To test the hypothesis that stomatal control of photosynthesis decreases during drier periods, we developed delta(13)C and delta(18)O chronologies spanning 1919-1999, and used a permutation regression approach to relate a 21-year running correlation between delta(13)C and delta(18)O to rainfall and temperature at Marble Bar and our study site. The relationship between delta(13)C and delta(18)O switched from being always negative before 1955 to being consistently positive after 1976, suggesting an increase in stomatal control of photosynthesis in recent decades. Changes in the delta(13)C-delta(18)O relationship reflected changes in rainfall, which has increased in the region by 30% since 1976. The correlation between delta(13)C and delta(18)O was positively related to the 21-year running mean of normalized rainfall anomalies at both the study site (P = 0.045, Adj. r(2) = 0.47) and Marble Bar (P = 0.046, Adj. r(2) = 0.48). In addition, the delta(13)C-delta(18)O correlation was negatively related (P = 0.047, Adj. r(2) = 0.61) to temperatures at Marble Bar. Our interpretation of the role of changes in climate affecting the relationship between tree-ring delta(13)C and delta(18)O is supported by evidence from the isotope composition of foliage samples: foliar delta(13)C and delta(18)O were negatively correlated with log stomatal conductance (delta(13)C, r = -0.41; delta(18)O, r = -0.42), whereas the correlation between foliar delta(13)C and delta(18)O was positive (r = 0.63, P = 0.027) after the summer wet period. Our data indicate that stomatal control of photosynthesis in Callitris adjusts to region-wide changes in climate and that, in a warmer and drier world, trees might adapt by increasing non-stomatal control of photosynthesis.