1. The relation between preganglionic activity and arterial tone was studied in preparations of bullfrog lumbar sympathetic ganglia 7-10 and the dorsal aorta. 2. Two or more stimuli evoked contractions when applied to the preganglionic C, but not the B pathway. Contractions were blocked when transmission in ganglia 9 and 10 was disrupted by cutting the sympathetic chain or adding (+)-tubocurarine. Contractions were antagonized by postganglionic action of guanethidine, but not by phentolamine or suramin. 3. Aortic responses to short trains (10-100 stimuli) were half-maximal at 0.3-0.5 Hz, saturated near 1 Hz and had a minimum latency of 8.9 s. By contrast, responses to 300 stimuli were half-maximal at 1 Hz and became 2.5-fold larger at 10 Hz. 4. Exogenous luteinizing hormone releasing hormone (LHRH) potentiated preganglionically evoked contractions. Endogenous LHRH mediated contractions evoked by 10 Hz stimulation in (+)-tubocurarine. These responses had a longer latency than in normal Ringer solution and were blocked by [D-pGlu1, D-Phe2, D-Trp3.6]-LHRH. The LHRH antagonist did not alter contractions evoked by continuous stimulation in normal Ringer solution or by bursts of stimuli in hexamethonium. 5. Exogenous neuropeptide Y (NPY) potentiated neurogenic contractions and responses to adrenaline. Benextramine blocked contractions produced by nerve stimulation, adrenaline and NPY, but not ATP. 6. The results show that contractions of the isolated aorta are tuned to physiological frequencies of activity in sympathetic C neurones. Peptidergic cotransmission in the ganglia can increase arterial tension, but not during synchronous activation of primary nicotinic synapses. It is suggested that the physiological role of LHRH arises from interactions with subthreshold nicotinic EPSPs and that postganglionic release of NPY shifts frequency tuning of the circuit during prolonged activity.