The effect of drug-induced hypertension on neurotransmitter release from dorsal raphe nucleus was studied by in vivo electrochemical electrodes in urethane anesthetized male Sprague-Dawley rats. Carbon paste electrodes were stereotaxically placed into dorsal raphe nucleus and neurotransmitter release was estimated electrochemically. Blood pressure was recorded from a femoral arterial catheter. Voltammograms taken from dorsal raphe nucleus showed two distinct peaks corresponding to norepinephrine and 5-hydroxyindole acetic acid (5-HIAA). After basal blood pressure and neurotransmitter release were monitored for 30 min, blood pressure was raised 50 mmHg by continuous intravenous infusion of L-phenylephrine hydrochloride. Drug infusion was discontinued after 50 min, but blood pressure and neurotransmitter release were measured for an additional 2 hr. Results showed that the 5-HIAA response increased immediately after the initiation of hypertension and remained elevated. By contrast, norepinephrine release initially decreased, then returned to the basal level and then rose in parallel with 5-HIAA to a level above baseline as drug-induced hypertension was discontinued. The same experimental protocol was used to study the electrochemical response to drug-induced hypotension. Blood pressure was lowered 20 mmHg by intravenous infusion of sodium nitroprusside dihydrate. During hypotension, no changes were seen in either transmitter response. However, as reflex hypertension appeared following discontinuation of the sodium nitroprusside infusion, the 5-HIAA response increased and the norepinephrine response decreased. These results show that drug-induced and reflex hypertension reduce norepinephrine release and increase serotonin turnover in dorsal raphe nucleus in anesthetized normotensive rats. These reciprocal changes appear to be a part of the neural response to hypertension.