The origin and extent of water deficits in Sitka spruce are discussed in relation to external conditions and the nature of the internal water transport pathway. Resistances to liquid water movement are calculated and compared with other resistances in the soil-plant-atmosphere continuum (SPAC). Under non steady-state conditions, plant resistances may be an important factor in controlling water deficit in the needles. The extent of stem shrinkage points to the presence of large stores of water which can buffer the effect of transpirational loss upon the level of water deficit in the needles by providing a source more readily available than that in the soil. Studies of the site of shrinkage suggest that water is stored by young unlignified cambial derivatives. The tissue water relations of Sitka spruce are investigated using the pressure chamber, and the pressure-volume curve is developed to generate equations which describe the variation of total water potential, solute potential, turgor potential, matric potential, bulk modulus of elasticity, and bound water content in relation to tissue water content. The water potential - relative water content curve (the water potential isotherm) shows that Sitka spruce has a high water retention capacity which enables low potentials to develop with only small losses of water content. The variation of solute potential with water content shows large deviations from the behaviour predicted by the Boyle-Van't Hoff relation. This results from large volumes of .matric or bound water, much of which appears to be held in the smaller interstices of the cell walls. The bound water content of twigs shows a large increase in autumn which coincides with the time when cold hardiness of photosynthesis develops in Sitka spruce.