The anatomical structure of the cross-field pits in unsieriate rays of Sitka spruce has been examined by scanning electron microscopy. Microscopic images were subjected to image analysis to explain the differences in radial permeability (as assessed by fluid uptake) of dried wood from selected trees of two different seed origins (Queen Charlotte Islands, QCI grown in Rhondda, South Wales vs. South Oregon, SO grown in Dalby, North-East England). Different features of ray pitting were observed. The most radially permeable seed origin (QCI) had considerably larger cross-field pits in latewood than the least permeable (SO). Variation in pit size across the growth rings was minimal in QCI but an abrupt increase in size was noted in the latewood of SO. It is concluded that one of the important anatomical features influencing preservative uptake is the size and number of the cross-field pits.