The variation of tension in a radial strand of a bordered pit membrane during drying has been derived theoretically from two approaches. One considers the mechanical extension of the strand, while the other considers the surface tension force caused by an annular liquid meniscus in the pit chamber. The tension has been calculated for pits in earlywood, in latewood and in regions near the earlywood-latewood boundary of a single growth ring in Pinus sylvestris L., using experimentally observed typical values for pit dimensions. The occurrence of aspiration of earlywood but not of latewood pits in air-seasoned wood is correctly predicted by the analysis, which also predicts that, contrary to accepted theory, earlywood pit membranes should be displaced and brought into contact with the pit border irrespective of the liquid present during drying. It is shown that this must involve considerable creep in the radial strands. Permanent aspiration must only occur when the liquid promotes bonding between the membrane and the border, and the probability that this is hydrogen bonding is discussed.