The amount of heat conducted to an isolated drop of water on a leaf is described by an equation which includes an effective thermal conductivity coefficient. Measurements of the dimensions of water drops on a horizontal wheat leaf give relationships which allow the volume and exposed surface areas to be obtained from drop diameter. These relationships are used in the experimental determination of the drop boundary-layer resistance and the effective thermal conductivity coefficient for drops on a leaf in a chamber. The boundary-layer resistance of the drop appeared to be independent of drop size and the mean value was about 60% of that for one side of the leaf. For drops with diameters less than 1 mm, conduction of heat to the drop reduced the leaf-to-drop temperature difference by over 50% of the value expected without conduction. Conduction of heat to drops will significantly affect the evaporation rate of surface water from cereal canopies after rain.