To assess the influence of sympathetic efferents on the dopamine function of carotid bodies, rats were exposed to long-term hypoxia (10% O2 in nitrogen for 1 or 3 weeks) after unilateral removal of the superior cervical ganglion. In the intact carotid bodies. long-term hypoxia increased the content and turnover of dopamine (DA). The dopaminergic response to hypoxia was reduced but not abolished by the ganglionectomy. To determine whether pre- or postganglionic sympathetic fibres are involved in the control of the dopamine function, rats were exposed to hypoxia either after unilateral transection of the preganglionic cervical trunk or after selective destruction of the postganglionic fibres by guanethidine. The preganglionic transection blunted the dopaminergic response to hypoxia whereas guanethidine had no effect. It is concluded that the sympathetic efferents may activate the synthesis and release of dopamine in glomus cells during long-term hypoxia. The sympathetic efferents responsible for the modulation of dopamine function are probably preganglionic fibres.