Acetal derivatives of hemicholinium no. 3 (HC-3) were synthesized and their chemical structures were confirmed by spectrophotometric evidence and elemental analysis. The acetals elicited a biphasic pattern of neuromuscular inhibition in the cat: (a) an immediate inhibition (early phase block) was judged to be curare-like as responses to acetylcholine (close i.a.) were abolished by acetal administration and reversal of inhibition was effected by neostigmine (25 micrograms/kg); (b) a slow, progressive inhibition of transmission (late phase block) was considered HC-3-like as it occurred only during high frequency stimulation and was antagonized by small doses of choline (1--3 mg/kg). In comparison to HC-3, significanlty greater curare-like activity was noted following acetal administration in vivo and in vitro. Mouse toxicity and cat nerve--muscle studies revealed the acetals to be HC-3-like but 1/2 to 1/3 as active as the parent compound. Cholinesterase inhibition by HC-3 and the acetals was low (I50 greater than 0.1 mM) and did not account for differences in activities. Acetal-elicited hypotension in the cat was attributed to postsynaptic ganglionic blockade and histamine release. Studies employing the 14C-(N-methyl) acetals furnished no evidence of the bioactivation (O-dealkylation) of the acetals to HC-3 in the cat in vivo or in cat liver in vitro. Chromatographic analysis afforded no evidence of molecular modifications of 14C-HC-3 or of the 14C-acetals in these systems.