In reaction-time (RT) tasks with unequally probable stimuli, people respond faster and more accurately in high-probability trials than in low-probability trials. We used functional magnetic resonance imaging to investigate brain activity during the acquisition and adaptation of such biases. Participants responded to arrows pointing to either side with different and previously unknown probabilities across blocks, which were covertly reversed in the middle of some blocks. Changes in response bias were modeled using the development of the selective RT bias at the beginning of a block and after the reversal as parametric regressors. Both fresh development and reversal of an existing response bias were associated with bilateral activations in inferior parietal lobule, intraparietal sulcus, and supplementary motor cortex. Further activations were observed in right temporoparietal junction, dorsolateral prefrontal cortex, and dorsal premotor cortex. Only during initial development of biases at the beginning of a block, we observed additional activity in ventral premotor cortex and anterior insula, whereas the basal ganglia (bilaterally) were recruited when the bias was adapted to reversed probabilities. Taken together, these areas constitute a network that updates and applies implicit predictions to create an attention and motor bias according to environmental probabilities that transform into specific facilitation.