The effects of the gamma-aminobutyric acid-B (GABAB) receptor agonist baclofen and the GABAA receptor antagonist bicuculline methiodide on the activities of the same neurons in the premotor cortex of two monkeys while they performed a visually guided task with a GO or NO-GO response were investigated. While bicuculline methiodide induced excitatory responses in all of the task-related neurons examined, baclofen induced inhibitory responses in 75% of the examined neurons and excitatory responses in 25% of the neurons. The results indicate that, in the monkey premotor cortex, not only GABAA but also GABAB receptors are involved in GABAergic modulation of neuronal activities related to the visually guided behavioral performance.