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CO2 exchange, transpiration and leaf water potential of Welwitschia mirabilis were measured in three contrasting habitats of the Namib desert. From these measurements stomatal conductance, internal CO2 concentration and WUE were calculated. In two of the three habitats photosynthetic CO2 uptake decreased and transpiration increased with increasing leaf age while in the third habitat CO2 uptake increased and transpiration decreased with leaf age. Except for the stomata of young leaf sections in this habitat, stomata closed with increasing dw leading to a pronounced midday depression of CO2 uptake. The high stomatal limitation of photosynthetic CO2 uptake of glasshouse-grown plants was verified in the natural habitat. Photosynthetic CO2 uptake saturated between 800 and 1300 mu mol photons m(-2) s(-1) depending on leaf age and habitat. CO2 uptake had a broad temperature optimum declining significantly beyond 32 degrees C. Predawn leaf water potential reflected water availability and atmospheric conditions in the three habitats and ranged from - 2.5 to - 6.2 MPa. There was a pronounced diurnal course of leaf water potential in all habitats. During the day a gradient in water potential developed along the leaf axis with the lowest potential at the leafs tip. With respect to whole plant balances of CO2 exchange and transpiration, there were marked differences between Welwitschias in the three habitats. Despite a negative CO2 balance over a period of five months, leaves in the driest habitat grew constantly at the expense of carbon reserves in the plant. Only at the wettest site did carbon gain exceed carbon demand for growth. The WUE of whole plants was insignificant in all habitats. The results were as contrasting as the habitats and plants and did not allow generalisations about adaptational features of Welwitschia mirabilis.