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We examined relationships between stable isotopes of carbon (delta C-13) and oxygen (delta O-18) 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 C-13 and delta O-18 chronologies spanning 1919-1999, and used a permutation regression approach to relate a 21-year running correlation between delta C-13 and delta O-18 to rainfall and temperature at Marble Bar and our study site. The relationship between delta C-13 and delta O-18 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 C-13-delta O-18 relationship reflected changes in rainfall, which has increased in the region by 30% since 1976. The correlation between delta C-13 and delta O-18 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 C-13-delta O-18 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 C-13 and delta O-18 is supported by evidence from the isotope composition of foliage samples: foliar delta C-13 nd delta O-18 were negatively correlated with log stomatal conductance (delta C-13, r = -0.41; delta O-18, r = -0.42), whereas the correlation between foliar delta C-13 and delta O-18 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.