Xylem pressure potential (P) and leaf surface resistance (R1) were measured for 3 different species which occupied different vertical positions in an oak-hickory forest. No rain fell during the 19-day, late summer study period, and diurnal patterns of P and R1 for white oak (Quercus alba L.), sugar maple (Acer saccharum Marsh) and sunflower (Helianthus strumosus L.) were greatly modified by decreasing soil moisture availability. Microclimatic factors varied vertically within the forest, and plant water status varied vertically in response to such factors. The most important of these was solar radiation, which probably caused higher leaf temperatures in the upper canopy and, thus, a steeper gradient of vapor pressure from the leaf to the air than was observed for the more shaded understory and ground vegetation.