Following intravenous administration of the cholinesterase reactivator HS-6 (30 mg/kg), blood pressure fell (up to 50 mmHg) and maximal blood levels of HS-6 reached 242 microgram/ml. HS-6 attenuated the pressor response resulting from carotid occlusion and the depressor effect of vagal stimulation. Doses of HS-6 below those used to protect against soman in different animal species (10--30 mumol/kg) progressively blocked the ganglion-stimulating effects of nicotine and dimethylphenylpiperazinium but not the pressor effect following adrenaline, a pattern similar to that produced by hexamethonium but only 1/84 as potent. HS-6, like hexamethonium and mecamylamine, progressively blocked the contraction of the nictitating membrane of the cat resulting from preganglionic stimulation. The results indicate that HS-6 possesses ganglion-blocking properties at doses likely to be used in the protection against soman poisoning. The ganglion-blocking properties of the drug may be a factor in the beneficial effects of HS-6.