For roots growing in anoxic media the intercellular pore-space of the root cortex provides a channel for the diffusive flow of respiratory gases between shoot and root. Three methods were used in an attempt to quantify the diffusive resistance of this pore space (Rp) in pea. Two of the methods were based on polarographic measurements of oxygen flow through the roots: the ‘cooling’ and ‘gas-mixture’ methods; the third was based upon measurements of porosity and root geometry (‘shape’). The last gave results which agreed closely with those obtained using gas-mixtures. The relationship between Rp and root length followed a curvilinear form in which there was little change in Rp in the earlier stages of growth (3 to 6.5 cm). The cooling method also produced a curvilinear pattern, but numerically the resistances were considerably higher. Also, resistance rose significantly in the shorter roots. It was found that respiratory activity in the roots at 3 °C could account for these discrepancies; an electrical analogue of the root programmed for resistance (gas-mixture/root geometry values) and 3 °C respiratory activity, gave results which matched those obtained by the cooling method. It was concluded that the gas-filled pore space in the pea root forms a relatively non-tortuous diffusion path and that resistance is very strongly influenced by root shape. As roots extend beyond a length of 7 cm there is an accelerated rise in Rp. An original intention, that of obtaining a measure of diffusive resistance across the root wall, was thwarted by the variability in the data.