Corticosteroids are potent modulators of human higher cognitive function. They are released in response to stress, and are thought to be involved in the modulation of cognitive function by inducing distinct rapid nongenomic, and slow genomic changes, affecting neural plasticity throughout the brain. However, their exact effects on the neural correlates of higher-order cognitive function as performed by the prefrontal cortex at the human brain system level remain to be elucidated. Here, we targeted these time-dependent effects of corticosteroids on prefrontal cortex processing in humans using a working memory (WM) paradigm during functional MRI scanning. Implementing a randomized, double-blind, placebo-controlled design, 72 young, healthy men received 10 mg hydrocortisone either 30 min (rapid corticosteroid effects) or 240 min (slow corticosteroid effects), or placebo before a numerical n-back task with differential load (0-to 3-back). Corticosteroids' slow effects appeared to improve working memory performance and increased neuronal activity during WM performance in the dorsolateral prefrontal cortex depending on WM load, whereas no effects of corticosteroids' rapid actions were observed. Thereby, the slow actions of corticosteroids seem to facilitate adequate higher-order cognitive functioning, which may support recovery in the aftermath of stress exposure. C orticosteroids are key modulators of human cognition. They are released in response to stress as the end product of the hypothalamic-adrenal-pituitary (HPA) axis, and are known to readily cross the blood-brain barrier to affect brain processing (1). Corticosteroids ensure sufficient energy supply to challenged tissues and control the excitability of neuronal networks, and are thereby thought to support and regulate the stress response (2). The hormones exert their actions upon binding of the mineralocorticoid (MR) and glucocorticoid receptor (GR), abundantly expressed in the brain (3-5). Recent animal research has indicated that receptor-binding causes both immediate nongenomic effects (6) and slow, genomic effects that manifest themselves several hours after stress exposure (7,8). By these distinct mechanisms, corticosteroids seem to influence neural plasticity in a timedependent manner (9).So far, most research on modulation of cognition has focused on medial temporal lobe structures, where corticosteroids have been shown to affect neuronal excitability, synaptic plasticity, and processes of memory retrieval and consolidation (10, 11). However, moderate to high levels of receptor expression in the prefrontal cortex (PFC) (5) make this structure susceptible to corticosteroid modulation as well. A current working hypothesis states that corticosteroids' rapid nongenomic effects work in concert with the effects of catecholamines during the early phase of the stress response (9, 12), and thereby optimize rapid adaptive behavior by reallocating neural resources away from higher-order cognitive processing regions in the PFC to promote vigilance, instinctive beh...