Preparations of leech muscle, 2 cm. long, were soaked in Ringer''s soln. 1/2 hr., fastened to the recording lever of a kymograph, and replaced in Ringer''s soln. for 15 min. A short base line curve was then recorded with the kymograph, and the preparations sensitized in 25 ml. of 1100,000 or 1:200,000 physostigmine soln. for 15 min. A second base line curve was then obtained. The standard contraction curves for solns. of acetyl-choline were obtained after soaking the leech preps. in a mixture of 10 ml. of the physostigmine soln. and 15 ml. of the acetylcholine soln. Twenty frog retinas light adapted for 3 hrs. with daylight were ground in a mortar with 15 ml. of 1:100,000 physostigmine soln. and 10 ml. Ringer''s soln. added. The testing of this soln. was carried out in the manner descr. with the leech preps. Twenty frog retinas completely dark adapted were tested in the same manner. The 20 dark adapted retinas contained about 150 y, and the 20 light adapted retinas, about 15 y acetylcholine.