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The spatial and diurnal variability of sap flow in a mature beech tree (Fagus sylvatica L.) was investigated on days with different climatic conditions (sunny, cloudy, rainy), during the summer. Sap flux density (v) was measured with six heat field deformation probes placed around the stem circumference. Each probe measured v at six sapwood depths. Daily v exhibited clear radial variation, and the shape of the radial profile of v differed substantially among circumferential positions. At some positions, daily v decreased monotonically towards the stem centre, whereas at others it showed an almost monotonic increase. Hence, the conducting sapwood area of the beech was highly asymmetrical. At all positions, conducting sapwood reached beyond the deepest sampled sapwood depth, precluding correct estimations of total sap flow. The radial profile of v also differed among measuring days. A general trend was that the inner sapwood contributed relatively more to total sap flow under better weather conditions. Besides variations among days, the shape of the radial profile of v also showed within-day variations. The contribution of the inner sapwood to total sap flow increased in the afternoon, with increasing vapour pressure deficit and photosynthetic active radiation. Because of large circumferential and temporal variability, no general function for the radial profile of v could be developed. [ABSTRACT FROM AUTHOR]