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Distribution of cholinesterase within the s i n u s node, AV node and His bundle of the human heart was studied utilizing acetylthiocholine as the substrate. Cholinesterase is present in large amounts in the conduction system tissue, much smaller amounts in right atrial myocardium and was not observed in ventricular myocardium. The sinus node artery was devoid of cholinesterase, as was the sarcolemma and nucleus of cells in the conduction system. Cholinergic nerve endings stained the most heavily of all tissue studied and were more numerous in the sinus node than the AV node. Stellate-shaped cells previously suggested to be the pacemaking site in the sinus node were found to contain abundant cholinesterase. Fibers in the AV nodal bypass tract, which lies between the AV node and right atrial endocardium, also contain cholinesterase. Within individual cells of the sinus node cholinesterase could be identified within myofibrils, with periodic absence in the linear staining most likely corresponding to the Z bands. Other organelles within the cells also contained cholinesterase, but their precise d e h i t i o n was impossible with the present technics. Possible functional significance of these histochemical observations were discussed.

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1. The objective of this study was to determine whether the chemical and biochemical changes induced by muscle wasting caused by dietary protein restriction are different in various skeletal muscles. 2. Rats were fasted for 3 d and then fed on a 10 g protein/kg diet for 21 d. Thirteen muscles from the trunk, forelimb, and hind-limb regions were analysed for muscle weight, and the content of water, fat, cellular and extracellular protein, DNA and RNA. Results were compared to values for an 'initial' control group killed at the start of the experiment. 3. Weight loss was greatest in trunk muscles and least in the distal forelimb muscles. Water content decreased in most muscles, but increased in three forelimb muscles. A significant loss of lipid was found in the gastrocnemius, while the biceps brachii gained lipid. Changes in lipid content of the muscles did not form a distinctive pattern. 4. All muscles except the distal forelimb muscles lost a significant amount of cellular protein, while all muscles except the diaphragm gained extracellular protein. 5. DNA content was unchanged in all muscles. The value for cellular protein:DNA was significantly reduced in the rectus abdominis and the diaphragm. A significant loss of RNA was found in all muscles; the percentage change was greatest in trunk muscles and least in the distal forelimb muscles. The values for RNA:protein and RNA:DNA were significantly lower in all muscles except two distal forelimb muscles. 6. With the exception of the water and lipid content of the muscles, the directions of the changes in the experimental animals were the same for all muscles. The results suggested, however, that the magnitude of changes in certain chemical and biochemical indices of composition may depend to some extent on the anatomical location of the muscle: trunk muscles tended to show the greatest percentage change, while the distal forelimbs changed the least.