This document describes two experiments on the effects of eserine and d-tubocurarine on acetylcholine-induced contractions of the frog rectus muscle. In the first experiment, eserine is added to the muscle bath and potentiates the effect of acetylcholine by inhibiting cholinesterases and allowing acetylcholine to accumulate at the motor end plate. In the second experiment, d-tubocurarine is shown to block the stimulatory action of acetylcholine on contractions of the frog rectus muscle when added before acetylcholine.