Stomatal conductance (gs) of bracken [Pteridium aquilinum] growing below a pine forest canopy was examined using a diffusion porometer. The variation in gs at different levels of pinnae insertion and both pinnae surfaces were examined. The response of gs to a range of conditions of radiation, atmospheric humidity deficit and soil moisture was also investigated. The range of soil moisture conditions experienced by the bracken was extended by establishing plots which were either irrigated in addition to normal rainfall or from which rainfall was excluded. On the lower surfaces of the pinnae g2 was consistently 3 times that on the upper surfaces. Of the lower pinnae surfaces gs declined with height in the plant but this trend was not shown by the upper pinnae surfaces. In response to increasing radiation, gs increased but tended to decline in response to increasing atmospheric humidity deficits when these occur at higher radiation levels. gs was highest in the irrigated plot and lowest in the dry plot, with intermediate values in the untreated bracken. Values of leaf water potential (.psi.) were not consistent with the different soil moisture levels and .psi. did not show a relationship with gs. Less than 50% of the variation in stomatal conductance could be accounted for by the variation in the measured environmental variables. Much of the remaining variation may be caused by short-term fluctuations in radiation at the understorey level. Stomatal conductance of the bracken understorey is less sensitive than that of the trees to atmospheric humidity deficits; this explains the increasing contribution to total forest transpiration from the understorey when dry atmospheric conditions prevail.