Cardiovascular and respiratory effects of intra-left atrial or intra-left ventricular injec-tion of serotonin were studied in conscious dogs (n = 8), anesthetized closed-chest dogs (n = 13) and anesthetized open-chest dogs(n= 9). Serotonin (50-200 /ig), injected as a bolus, resulted in an initial bradycardia and hypotension followed by a delayed tachycardia and hypertension in the conscious dogs. The hypertension was seen as an increase of 21.5 ± 2.7 (mean ± SE) nun Hg from a control pressure of 102.5 ± 1.9 mm Hg, whereas the initial decrease in pressure was 22.6 ± 1.9 mm Hg. The tachycardia was 23.3 ± 3.9 beats/min above a control heart rate of 104.9 ± 3.9 beats/min whereas the bradycardia was 58.5 ± 3.7 beats/min below control. There was a significant attenuation of the hypotension in both groups of anesthetized dogs. In fact, no hypotension was elicited in the open-chest anesthetized dogs. Open-chest anesthetized dogs showed only a hypertensive response (mean increase 67.2 ± 5.5 mm Hg). Stimulation of respiration was seen in all groups of dogs. In conscious dogs there was a 214.8 ± 15.4% increase in respiratory depth and a 20.8 ± 3.1 breaths/min increase in respiratory rate. Atropine significantly reduced the bradycardia and abolished the hypotension in conscious dogs. Bilateral cervical vagotomy did not abolish the response in open-chest anesthetized dogs. We conclude that the so-called "hypertensive coronary chemoreflex" is altered dramatically by the state of the preparation and by anesthesia.Circ Res 47: 509-515, 1980