Spatial variation in carbon isotope composition (delta(13)C) within the crown of an individual tree complicates sampling strategies, but a systematic study has allowed constraining factors to be determined. Spatial and temporal variations of the delta(13)C of Pinus sylvestris L. needles were investigated on three exposed, south-facing, branches over 17 months ( two growing seasons). A positive relationship (about 2% m(-1)) was found between needle delta(13)C and trunk-needle distance on the branch. Abaxial needles were characterized by less negative delta(13)C values (0.5parts per thousand) compared with adaxial needles. Both effects were interpreted in terms of branch hydraulic conductivity including the presence of compression wood. A close relationship was found between delta(13)C measured in needles and in adjoining branch wood. Correcting the monthly data for spatial variation, a transient increase in needle delta13C during spring was detected (about 0.6parts per thousand), whereas changes in needle delta(13)C during summer, autumn and winter were minor and positively related to solar radiation.