Preliminary pharmacological screening of twelve styrylpyridine analogues with specific and/or selective in vitro choline acetyltransferase (ChAc) activity was undertaken. Observations were made on acute toxicity and the behavioral and neurological profile after sub-lethal doses in mice and rabbits. The neuromuscular effect of the compounds was investigated both in vivo and in vitro. The compounds caused a non-depolarization or a biphasic depolarization block. The block caused by bisquaternary or monoquaternary ChAc inhibitors in vitro could be readily reversed by repeated washing with fresh bathing solution; in contrast the block caused by the tertiary inhibitors, e.g. 4-(1-naphthylvinyl)-pyridine (compound III), was more difficult or impossible to reverse. All compounds investigated decreased the twitch tension of the directly stimulated muscle. Neuromuscular blocking doses of compoundsIII caused a transient but severe fall of blood pressure and slowing of the respiratory rates in cats. On the basis of these preliminary studies the pharmacological effects of the compounds investigated cannot be attributed to their ChAc inhibitory effect alone. It seems probable that other yet unclarified effects of these compounds may partly or wholly be responsible for their pharmacological effects.