Some new aryl and dichloroacetyl esters of tropine and ψ-tropine have been studied with respect to their toxic properties on intravenous administration to mice and cats, in terms of their dual roles as potentiating and/or blocking agents in isolated neuromuscular preparations, and with regard to their actions on the purified acetylcholinesterase-acetylcholine system. In mice the aryl esters display toxicity patterns that fit an empirical structure vs. potency correlation in the form of linearity between (acute) log LD50 values and an index of electronegativity (σ*) of the esterifying group, when the index exceeds a minimum level of + 0.40. The dichloroacetyl esters fail to conform to the same relationship, in part because of their ease of solvolysis in aqueous medium. On the rat phrenic nerve-diaphragm preparation all the new esters obey the general series pattern of moderate amplification of twitch response amplitudes at low incubating concentrations of agent, which is superseded abruptly by twitch blockade as the concentration is further raised. This preparation displays high stereospecificity in response to isomeric esters with tropine vs. ψ-tropine configurations, oriented in favor of tropine structures which are similarly more potent in intact animal work. With the esterase system, stereospecific response toward structure of the new aryl esters acting as reversible inhibitors is again high, but with a pattern (ψ-tropine esters more potent than tropine isomers) that is just the reverse of that seen with toxic actions in intact animals and the rat nerve-diaphragm. The correlation between esterase-inhibitory strength and electronegativity of the esterifying residue is also inverted with respect to that found for acute toxic response in the mouse, with relative electron-supply toward the -OCO- ester function aiding the esterase binding process. These results are discussed in the framework of potential ester-chemoreceptor interactions underlying the observed toxic manifestations. © 1964.