We evaluated the performance of the Heat Dissipation Technique (HDT) to measure sap flow in whole trees by comparison with potometric water uptake. Two tropical lowland species, Ochroma lagopus (balsa), a pioneer species with light wood and Hyeronima alchorneoides (pilón), a late-successional species with hard wood were examined. Diurnal courses of sap flow measured with the HDT showed good agreement with potometry. At the low sap flow rates (below 1 Kg h-1) occurring during nocturnal recharge HDT consistently underestimated sap flow rates. This resulted in the failure of the current version of the HDT to measure nocturnal water uptake, an important component of the water budget of at least one of the two species examined.