1 In pithed cats, the spinal sympathetic outflow was stimulated preganglionically at segments C7 and TI and heart rate responses and nictitating membrane tone were measured in parallel.2 Clonidine and a related drug, BS 100-141 (N-amidino-2-(2,6-dichlorophenyl)acetamide hydrochloride), caused a dose-dependent inhibition of the stimulation-induced tachycardia but did not inhibit responses of the nictitating membrane. The inhibition of heart rate was antagonized by the oa-adrenoceptor blocking drug, phentolamine. 3 In isolated hearts of rabbits, noradrenaline release in response to adrenergic nerve stimulation was reduced by clonidine and BS 100-141 and the effect was antagonized by phentolamine. 4 The results support the view that presynaptic ci-adrenoceptors are involved in the regulation of transmitter release from adrenergic nerves. Cardiac adrenergic nerves appear more sensitive to a-adrenoceptor-mediated inhibition of impulse transmission than the sympathetic nerves to the nictitating membrane.