Abstract Recently, it has been reported that tetramethylammonium chloride causes contraction of acetylated glycerol-treated muscle fibers while small metallic cations cause little or no contraction ( Bowen and Martin, 1964). This led to speculation concerning the effect of large cations on the ATP-induced contraction of normal glycerinated muscle fibers. Several large cations have been tested and will be discussed in forthcoming publications. The present report concerns the effect of tetramethylammonium ion (TMA) and carnosine on the ATP-induced contraction of glycerinated muscle fibers. Carnosine is a constituent of vertebrate muscle and exists in rabbit to the extent of about 0.6 percent ( Severin, 1964).