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Dietary intake of polyunsaturated fatty acids (PUFAs) is crucial for neuronal functions, can positively affect cognition, and reduce glucocorticoid (e.g. corticosterone) concentrations in response to stress. We investigated the effects of walnut oil high in PUFAs on spatial cognition and fecal corticosterone metabolite (FCM) concentrations under non-stressed conditions in rats. Unexpectedly, PUFA-supplemented rats had higher FCM concentrations and elevated concentrations generally impaired learning in the subsequent T-maze task. Statistically adjusting for individual FCM concentrations, however, revealed that learning performance was improved in PUFA-supplemented rats. The results suggest that glucocorticoids can modulate the effects of PUFAs on spatial learning under normal (non-stressed) conditions and call for consideration of basal physiological conditions in spatial learning tasks.