Ionophoretically applied acetylcholine and vagal stimulation in the arrested sinus venosus of the toad, Bufo marinus.

1. The effects of acetylcholine (ACh), applied by ionophoresis, on the isolated arrested sinus venosus of the toad, Bufo marinus, were examined. 2. At each position where ACh was applied across the surface of sinus venosus preparations, a hyperpolarization was produced. These responses were abolished by hyoscine, indicating that muscarinic cholinoceptors are widely distributed over the surface of these muscle cells. 3. Vagal stimulation produced hyperpolarizations which were mimicked, to some extent, by ionophoretically applied ACh. 4. The responses to ionophoretically applied ACh were abolished by adding barium ions to the perfusion fluid, whereas responses to vagal stimulation persisted. 5. The responses to ionophoretically applied ACh were consistently slower than those to vagal stimulation. It is argued that the pathways activated by neural and applied ACh have different kinetics of activation.