Technical Abstract: Peanut (Arachis hypogaea L.) is able to withstand periods of water scarcity either in the early or late periods of the growing season, but suffers significant stress and yield loss during drought periods in mid-season, or the period coinciding with peak flower production and pod maturation. In fact, periods of early drought stress occurring just prior to primary flower production have been shown to up-regulate physiological function and cause an acclimation response that allows the crop to better tolerate subsequent mid- or late-season drought stress. However, the optimum duration of early season drought stress is not known, such that an acclimation response is invoked but the crop is not detrimentally stressed. To determine the optimum duration of early season water deficit, 50% of full irrigation (ET replacement) was applied for 0 (control), 10, 20, 30, 40, and 50 days after stand emergence, with full irrigation applied for the duration of the growth season subsequent to each stress period. To examine the possible genetic variability in acclimation response, two peanut genotypes were chosen: one drought susceptible (Georgia Green: GG) and the other drought tolerant (C7616). At each ten day stress period, physiological responses were measured both in a dry condition (three days without irrigation) and in a recovered condition (24 hours after irrigation). Differential strategies of withstanding drought appeared to be operating in the two genotypes. C7616 exhibited an acclimation response in both 10 and 40 day deficit treatments by maintaining physiological function in a dry condition above the control (fully irrigated) throughout most of the season. This acclimation ability was absent in the drought susceptible genotype, GG, which was detrimentally affected by all deficit time period treatments. C7616 exhibited a two stage strategy to tolerating drought: a quick response to drought through modification of physiological function (10 day treatment); and further changes in function with increasing duration of drought (40 day treatment). This ability to respond plastically to both short and long term drought may be part of the physiological foundation of drought tolerance in peanut.