Microinjections of low doses of the potent and selective cannabinoids WIN 55,212-2 and CP 55,940 into the lateral ventricle produce long-lasting reduction in sensitivity to noxious thermal stimuli (1). To determine the central distribution of ventricularly administered WIN 55,212-2, we microinjected an analgesic dose of the drug with [3H]WIN 55,212-2. At the peak time of antinociception, the radiolabeled drug was confined to periventricular sites throughout the brain. The contribution of particular periventricular structures to the antinociceptive effect was evaluated using intracerebral microinjection techniques and the tail-flick test. Guide cannulae were implanted above the following periventricular structures: the medial septal area, lateral habenlua, perihypothalamic area, arcuate nucleus of the hypothalamus, dorsal raphe nucleus and the dorsolateral and ventrolateral aspects of the periaqueductal gray. Microinjections of WIN 55,212-2 (5 micrograms/0.5 microliter) into the medial septal area, lateral habenula, perihypothalamic area, arcuate nucleus, and ventrolateral periaqueductal gray did not significantly affect tail-flick latencies. By contrast, microinjections of WIN 55,212-2 into the dorsolateral periaqueductal gray and the dorsal raphe significantly elevated tail-flick latencies. The results of this study indicate that at least two periventricular structures within the brain are involved in cannabinoid antinociception.