Effect of 6 Gardneria alkaloids, namely, gardneramine, gardnerine, gardnutine, hydroxygardnutine, 18-demethylgardneramine, and Alkaloid I on ganglionic transmission was examined in the rabbit and rat superior cervical ganglionic in situ preparations. In general, no marked, if at all, difference between both preparations with respect to their maximal response to each of the first 4 compounds was observed. Among these compounds, the most potent ganglion blocking effect was found in gardneramine, gardnerine, and Alkaloid I. Their effect was short-acting compared with that of hexamethonium. The activity of gardneramine and gardnerine was about 1/2 (rabbit), and about 1/4 (rat), as potent as that of hexamethonium. On the other hand, the effect of hydroxygardnutine and 18-demethylgardneramine was very weak. Concerning the pharmacology of Gardneria alkaloids which have chemically been investigated by Sakai, et al., general pharmacological screening of gardneramine (GA) and gardnerine (GI), effect of GA on ganglionic transmission in the cat superior cervical ganglion, and effect of GA, GI, gardnutine (GN), and hydroxygardnutine (HG) on neuromuscular transmission in the rat limb have been reported. In the present study, as GA showed a hexamethonium (C6)-like action on the cat superior cervical ganglion and as the ganglion blocking effect of C6 on the dose basis was more potent in the cat than in the rat, effect of 6 Gardneria alkaloids on ganglionic transmission in the rabbit and rat superior cervical ganglia in situ was examined for comparison of the activity among the compounds and from the viewpoint of species difference.