Both serotonergic and cholinergic mechanisms in the hypothalamus are involved in blood pressure control. Because lesion of the dorsal raphe or median raphe nucleus decreases the turnover of hypothalamic acetylcholine, interactions between serotonin and acetylcholine have been investigated in the hypothalamus with respect to blood pressure. Pharmacological manipulations that interfere with cholinergic neurotransmission in the posterior hypothalamus block the pressor response to serotonin in the anterior hypothalamic/preoptic area (AH/PO), reduce the pressor response to dorsal raphe stimulation, and have no effect on the pressor response to median raphe stimulation. Administration of the serotonin antagonist metergoline into the AH/PO blocks the pressor response to serotonin, slightly reduces the response to dorsal raphe stimulation, and does not affect the pressor response to median raphe stimulation. It is concluded that a hypothalamic cholinergic mechanism is needed to enable hypothalamic serotonergic neurons to elevate blood pressure. The dorsal raphe nucleus may be a source of these serotonergic terminals involved in this pressor response.