For an “in situ” thermocouple hygrometer to be of value it must give reasonable estimates of plant-water potential and must respond rapidly to plant-water potential changes. Leaf thermocouple hygrometers (Wescor) and specially fabricated stem thermocouple hygrometers were evaluated on peanut (Arachis hypogaea L.) plants under well-watered and drought conditions in a growth chamber. When soil-water stress was low and plant-water movement was near steady state, the two sensors gave similar water potential values. When soil-water stresses were imposed or when plant process varied cyclically (eg., photosynthesis, transpiration), stem hygrometers sensed dynamic changes in the plant's water potential more consistently than did leaf hygrometers placed on leaves with intact cuticles. It appears that both the stem and leaf hygrometers hold promise for sensing plant water potential changes of peanut in the field.