In halothane-anaesthetized, vagotomized, SA-denervated rats, the activity of various sympathetic nerves has been analyzed with respect to phrenic nerve discharge (an indicator of central respiratory drive (CRD)). The cervical and lumbar sympathetic nerves had maximal activity following, and were least active during phrenic nerve discharge. In contrast, the splanchnic, cardiac, renal and adrenal nerves exhibited their activity peak during phrenic nerve discharge (i.e. inspiration). Similar activity profiles were observed after ganglion blockade in the mixed pre- and postganglionic fibre preparations. These observations indicate that it is the subpopulations of preganglionic neurones and the proportional contribution of each to whole-nerve activity which give rise to the differences in CRD-related activity profiles between nerves.