High nocturnal transpiration rates (5-15% of total water loss in terrestrial plants) may be adaptive under limited fertility, by increasing nutrient uptake or transport via transpiration-induced mass flow, but the response of stomata in the dark to environmental variables is poorly understood. Here we tested the impact of soil phosphorous (P) concentration, atmospheric CO2 concentration and air temperature on stomatal conductance (g(s)) during early and late periods in the night, as well as at midday in naturally, sun-lit glasshouse-grown Eucalyptus tereticornis Sm. seedlings. Soil P was the main driver of nocturnal g(s), which was consistently higher in low soil P (37.3-79.9 mmol m(-2) s(-1)) than in high soil P (17.7-49.3 mmol m(-2-1)). Elevated temperature had only a marginal (P = 0.07) effect on g(s) early in the night (g(s) decreased from 34.7 to 25.8 mmol m(-2) s(-1) with an increase in temperature of 4 degrees C). The effect of CO2 depended on its interaction with temperature. Stomatal conductance responses to soil P were apparently driven by indirect effects of soil P on plant anatomy, since g(s) was significantly and negatively correlated with wood density. However, the relationship of g(s) with environmental factors became weaker late in the night, relative to early in the night, likely due to apparent endogenous processes; g(s) late in the night was two times larger than g(s) observed early in the night. Time-dependent controls over nocturnal g(s) suggest that daytime stomatal models may not apply during the night, and that different types of regulation may occur even within a single night. We conclude that the enhancement of nocturnal g(s) under low soil P availability is unlikely to be adaptive in our species because of the relatively small amount of transpiration-induced mass flow that can be achieved through rates of nocturnal water loss (3-6% of daytime mass flow).