The intravenous LD 50 in rats and I 50 to sheep AChE of several compounds of the type (RO) 2PO·S·CH 2·CH 2·Z have been determined. Z represents SR, RR' or, in one case, R 3, where the R's are alkyl groups. Special precautions had to be taken to get reliable values. The LD 50 and I 50 of the cationic compounds are very much lower than those of their uncharged analogues. Two of the sulphonium compounds are the most toxic phosphorus anti-AChE's reported. The results are discussed in terms of the mechanism of inhibition at the enzyme surface.