A method is described which allows the determination of the average size, and the average number per tracheid, of the pit membrane pores through which fluids can pass in the longitudinal direction along oven-dried conifer wood. This consists in the observation of flow rates through the wood, in turn, of a non-polar liquid (n-hexane) and of a gas (air) at low pressure. Preliminary tests with Millipore filters of known porosity show that the method is sufficiently reliable to give pore sizes with acceptable accuracy, and reasons are given to expect the flow equations derived to be applicable to flow through wood. For all species examined the radii of the pores are larger for sapwood than for heartwood. They vary with species, ranging from 0$\cdot $34 $\mu $m (Larix leptolepis, heartwood) to (exceptionally among the species examined) 2$\cdot $75 $\mu $m (L. leptolepis sapwood) with an average of about 0$\cdot $8 $\mu $m. The average number of pores per tracheid varies with species but is always of the order of unity. This is lower than was expected; it is, however, in harmony with other recent work and indicates a widespread aspiration of pits during drying. The precise meaning of the average pore size determined by the present methods depends on the distribution of pore sizes in a specimen and this is examined. The method used makes the assumption implicit in all previous work, that the resistance to flow resides entirely in the pit membrane. It is shown that, if the figure of unity means literally that each tracheid in a specimen possesses one open spore, this assumption is valid. If, however, only a few tracheids are available for flow, with correspondingly a greater number of open pores per tracheid, then this assumption cannot be accepted. Further work is necessary to determine the number of tracheids involved.