The LD50 of subcutaneously-injected sarin (GB: isopropyl methylphosphonofluoridate) in mice was 172 micrograms kg-1. Mice were treated with sarin at doses between 25 and 150 micrograms kg-1, administered subcutaneously. After sacrifice of the animals, the diaphragms were removed and stained for acetylcholinesterase activity and the presence of ionized calcium. Calcium was found in the diaphragms of those mice to which sarin had been administered at doses of 50 micrograms kg-1 or above. Calcium accumulation was not present in diaphragms from those animals that had received 25 micrograms kg-1. Calcium accumulation occurred earliest and remained longest in diaphragms from those animals receiving the highest doses. Accumulation of calcium was associated with end-plates, as demonstrated by an acetylcholinesterase histochemical method.