Pressure transducers were attached to twigs of orchard trees and potted trees of walnut (Juglans regia L.) to measure winter stem xylem pressures. Experimental potted trees were partially defoliated in the late summer and early autumn to lower the amount of stored carbohydrates. Potted trees were placed in cooling chambers and subjected to various temperature regimes, including freeze-thaw cycles. Xylem pressures were inversely proportional to the previous 48-h air temperature, but positively correlated with the osmolarity of the xylem sap. Defoliated trees had significantly lower concentrations of stored carbohydrates and significantly lower xylem sap osmolarities than controls. Plants kept at 1.5 degrees C developed xylem pressures up to 40 kPa, just 7% of the theoretical osmotic pressure of the xylem sap. However, exposure to low, nonfreezing temperatures followed by freeze-thaw cycles resulted in pressures over 210 kPa, which was 39% of the theoretical osmotic pressure. A simple osmotic model could account for the modest positive winter pressures at low, nonfreezing temperatures, but not for the synergistic effects of freeze-thaw cycles.