1. The pharmacology of suxamethonium blockade of the rat phrenic nerve-diaphragm preparation has been studied in the presence of some foreign anions.2. Blockade proceeded in two distinct phases in both normal and modified Krebs solution.3. In normal Krebs solution the characteristics bore no close resemblance to either depolarizing or competitive type blockade.4. In the presence of foreign anions the characteristics of the blockade more closely resembled those of depolarization.5. There was an increase in the sensitivity of the motor end-plate region of the muscle to the depolarizing action of acetylcholine, carbachol and suxamethonium in the presence of the anions.6. Although the anions enhanced the depolarizing activity of suxamethonium, the blocking potency of the drug was unaltered.7. It is suggested that end-plate depolarization plays little part in suxamethonium blockade of the isolated rat diaphragm and that desensitization is the primary cause of the blockade.