The exercise ventilatory response is augmented during conditions of increased respiratory dead space (delta Vd), a phenomenon that we refer to as short term modulation (STM). To test the hypothesis that serotonin is necessary in the mechanism underlying STM, experiments were conducted on ten awake goats. Ventilation, CO2 production and PaCO2 were measured at rest and during treadmill exercise (4 km/h, 5% grade), with and without delta Vd (0.25 L), before and after systemic administration of the serotonin receptor antagonist, methysergide maleate (n = 6; 1 mg/kg, i.v.), or the tryptophan hydroxylase inhibitor, p-chlorophenylalanine (PCPA; n = 4; 100 mg/kg, i.v.). Pre-methysergide: (1) PaCO2 decreased from rest to exercise to a similar degree with (-1.9 mmHg) and without (-1.8 mmHg) delta Vd; (2) the exercise ventilatory response increased 59% +/- 13% (P < 0.01) with delta Vd, accounting for similar exercise PaCO2 regulation and demonstrating STM; and (3) effects of delta Vd on exercise tidal volume and frequency responses were inconsistent. Post-methysergide: (1) there were no significant effects on ventilation or PaCO2 at rest or during exercise in control (mask) conditions; (2) the exercise ventilatory response was unaffected by delta Vd, thereby allowing PaCO2 to increase 4.1 +/- 3.0 mmHg from rest to exercise (P < 0.05); and (3) with delta Vd during exercise, the tidal volume response was increased, but was offset by a decreased frequency response. Following PCPA (16-24 h): (1) hyperventilation was evident at rest and during exercise; (2) the exercise ventilatory response was augmented, indicating STM; and (3) the exercise ventilatory response with delta Vd was not affected further, allowing PaCO2 to increase from rest to exercise and indicating an inability to elicit further STM. These data suggest that serotonin is necessary for short term modulation of the exercise ventilatory response with increased respiratory dead space, although the location of relevant serotonin receptors is not yet clear.