A method for determining the mass flow rate of xylem water in thin stems under natural field conditions is presented. Diurnal courses of xylem water flow and stomatal conductance of the vines Entadopsis polystachya Cyclanthera multifoliolata and Serjania brachycarpa were examined in a tropical deciduous forest on the west coast of Mexico. E. polystachya (leaf area 23.6 m2) had a maximum water flow rate of 6.50 kg h-1 or 1.44 kg cm-2 stem basal area h-1; daily water use was 2.00 kg m-2 leaf area day-1. S. brachycarpa (leaf area 4.5 m2) and C. multifoliolata (leaf area 3.6 m2) had a maximum water flow rate of 0.72 and 0.19 kg h-1 or 0.63 and 0.92 kg cm-2 stem basal area h-1. Daily water use was 1.26 and 0.39 kg m-2 leaf area day-1 respectively. The daily courses of xylem water flow were strongly influenced by the orientation of the leaf area to irradiance and its intensity. While leaves of E. polystachya had a constant high stomatal conductance during the day S. brachycarpa had a maximum stomatal opening in the morning followed by continuous closure during the rest of the day. In contrast to the woody species the herbaceous C. multifoliolata exhibited a strong midday depression of stomatal conductance and wilting of its leaves. The leaf biomass accounted for 8% (Entadopsis) 16% (Serjania) and 23% (Cyclanthera) of above-ground biomass. The relation of sapwood area to leaf area supplied (Huber value) was 0.19 (Entadopsis) 0.18 (Serjania) and 0.06 (Cyclanthera) cm2 m-2.