1) The author investigated whether presynaptic alpha-adrenoceptors exist in the cardioaccelerator nerves of cold-blooded animals(frog, tortoise) as in ones of in mammals. 2) Each atrial preparation of a frog, tortoise and guinea-pig produced the positive chronotropic and inotropic responces to field stimulation. Each ventricular muscle preparation of frog and tortoise produced positive inotropic responces to field stimulation. 3) Both the responces of frog atrium and the inotropic response of frog ventrice to the stimulation were abolished or markedly inhibited by the presence of tetrodotoxin, guanethidine and proparanolo. Both responses of tortoise atrium to the stimulation were markedly inhibited by propranolol and the inotropic response ventricle to the stimulation was markedly inhibited by tetrodotoxin. 4) Both responses of frog and tortoise atrium, and the inotropic response of frog and tortoise ventricle to the stimulation were not affected by clonidine and yohimbine. 5) Both responses of guinea-pig atrium to the stimulation were markedly inhibited in the presence of clonidine and this clonidine-induced inhibition was not observed in the presence of yohimbine. 6) The above results suggest that presynaptic alpha-adrenoceptors do not exist in the cardioaccelerator nerves of frog and tortoise, being different from those of mammalisn animals.