The contribution of sympathetic and vagal inputs to ventilatory depression induced by dopamine was studied in eighteen anaesthetized, spontaneously breathing, normoxic cats. Breathing was via a tracheostomy. Dopamine (20 micrograms (kg body wt)-1) was injected intravenously in the intact animal, then after section of the cervical sympathetic trunks, and finally after midcervical vagotomy. Dopamine, injected as a bolus, induced depression of ventilation, affecting predominantly the volume component of the breathing pattern at all experimental stages. The extent of volume reduction was larger and different from that in intact animals following section of sympathetic (P < 0.05) and vagal trunks (P < 0.01). The respiratory cycle was significantly prolonged (P < 0.01) prior to vagotomy, due entirely to the increase in the expiratory time (TE). Bilateral section of the carotid sinus nerves performed in six cats virtually abolished postdopamine ventilatory depression.