1. Experiments were designed to evaluate the effect of converting enzyme inhibitors on autonomic nervous system function in the rat. 2. Arterial blood pressure, heart rate, efferent postganglionic sympathetic activity and afferent nerve activity from the right renal nerves were recorded in anaesthetized, spontaneously breathing, non diuretic rats, either with an intact spinal cord or with a spinal cord transected at the T6 level, before, during and after intravenous injections of 0.5--1.0 mg/kg of teprotide or captopril. 3. After injection of drugs, efferent sympathetic nerve activity markedly increased and reached its peak value 4 min later, both in rats with an intact spinal cord (101 +/- 21% mean +/- SE, above the control discharge) and with a spinal cord transected at the T6 level (166 +/- 41%, above the control). 4. Afferent activity from the renal nerve, on the other hand, did not consistently change during converting enzyme blockade. 5. The results indicate that the efferent sympathetic excitation cannot be due either to a baroreceptor or to a renorenal reflex. This excitation might be responsible, at least in part, for the increase in renin secretion which follows the blockade of angiotensin-converting enzyme.