Phenylbiguanide (PBG), a 5-HT₃ (serotonin) receptor agonist, has been used in many studies as a "selective" agonist to elicit reflex bradycardia and hypotension through activation of cardiac and pulmonary vagal afferents. Because we have shown that endogenous 5-HT stimulates ischemically sensitive abdominal sympathetic afferents through 5-HT₃ receptors, we investigated the possibility that left ventricular (LV) and intra-arterial administration of PBG may evoke a competing reflex response by increasing the activity of sympathetic visceral afferents in anesthetized cats. Mean arterial pressure (MAP) and heart rate (HR) were monitored. When both vagal and sympathetic afferents were intact, PBG (40 µg/kg, injected into the LV) significantly decreased MAP and HR in 8 of 10 cats but increased MAP in the remaining 2 cats. After bilateral cervical vagotomy, LV PBG significantly increased MAP. PBG (40 µg/kg ia) significantly increased MAP and HR, whereas intravenous PBG significantly decreased MAP and HR (n = 10 cats). Furthermore, the pressor response to PBG (40 µg /kg ia) was reduced by 68% (P < 0.05; n = 4 cats) by celiac and mesenteric ganglionectomies. In studies of single-unit abdominal sympathetic afferents, intra-arterial but not intravenous PBG (40 µg/kg) significantly increased activity of 10 ischemically sensitive afferents but not ischemically insensitive afferents. Blockade of 5-HT₃ receptors with tropisetron (200 µg/kg iv) eliminated the response of the afferents and the pressor response to PBG. These data indicate that PBG administered into the LV usually, but not always, evokes a depressor response that is converted to a pressor response following cervical vagotomy. Also, intra-arterial PBG induces a pressor response by stimulating 5-HT₃ receptors largely associated with ischemically sensitive abdominal sympathetic afferents.