1. The relation between the concentration of edrophonium in plasma, inhibition of red cell acetylcholinesterase, and neuromuscular transmission was studied in the rat. 2. In both in vivo and in vitro conditions, red cell acetylcholinesterase activity was predictably related to the concentration of the quaternary amine. 3. After low doses of edrophonium (1.0 mumol/kg) there was a significant correlation between the monophasic potentiation of twitch tension and the plasma concentration of the drug. In contrast, with higher doses of edrophonium (4.0 or 10.0 mumol/kg) a biphasic potentiation of twitch tension was observed; this was only correlated with the plasma concentration of the drug during the secondary decline in neuromuscular facilitation. Subsequent recovery of normal neuromuscular transmission invariably occurred at a constant plasma concentration of edrophonium.