Because of the suppressive effect of stress on reproduction and health, it is important to evaluate potential stressors that may compromise captive breeding programs for endangered species. However, behavioral and physiological measures of stress are sometimes difficult to interpret, and their relationship to stress can be obscured by factors unrelated to stress, such as seasonal and diurnal patterns. Here we present findings for daily and seasonal variation in glucocorticoid (GC) secretion and behavior from a 6-year study of 2 giant pandas (Ailuropoda melanoleuca). In the American black bear (Ursus americanus), seasonal patterns of corticoid secretion are putatively linked to metabolic demands of hibernation. Although pandas do not hibernate, we have found a similar pattern of GC dynamics. Using radioimmunoassay of urinary GC metabolites, we found seasonal variation in GC levels in an adult female and an adult male panda. As in black bears, winter and spring GC levels were significantly higher than summer levels. Additionally, in the female, GC levels during the period of parental care and lactation were higher, regardless of calendar season, than during other periods. Diurnal patterns were also detected in both the female and male panda, with elevated GCs in the morning sample. However, these diurnal patterns were not evident during the fall and summer months. Daily levels of several behaviors potentially indicative of stress also varied significantly with season in both male and female. Additionally seasonal dynamics of feeding behavior were documented. In the female, periods of elevated and diminished appetite were associated with embryonic diapause and post-implantation, respectively. The male had elevated feeding during the fall. Although these patterns are instructive for comparison with other species, we urge caution because our limited sample size does not allow us to extrapolate beyond the individuals studied.