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We examine seasonal and interpopulational variation in field levels of corticosterone (B) in the lizard Uta stansburiana. Corticosterone is the primary reptilian glucocorticosteroid, and elevated levels of this hormone have been correlated with physiological stress under both field and laboratory conditions. Patterns of seasonal variation in plasma B were generally similar in males and females, with peaks coinciding with periods of peak reproductive activity (males) or peak reproductive condition (females). Levels of circulating B usually declined as the activity season progressed and were lowest in fate summer, especially among males at desert sites-including our southernmost site, where lizards exhibited poor body condition as a result of drought. Variation in B levels among populations was probably attributable to variation in the timing of seasonal samples (e.g., in relation to peaks in reproductive activity). Apparently these populations did not experience serious (i.e., debilitating) physiological stress, as the highest recorded levels were lower than those of captive-stressed U. stansburiana. This suggests that corticosterone levels simply parallel annual patterns of activity, especially reproduction. These findings agree with other studies that have indicated a positive, rather than a negative association between glucocorticosteroid secretion and reproductive activity. These results also suggest that basal (baseline) levels of B in free-living animals may not be indicative of the degree of ’’stress.’’