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.