Evidence for differences between the periurethral striated muscle and external anal sphincter of the cat is provided by measurement of the contractile properties in situ and in vitro and the electrophysiologic properties in vitro. In the periurethral striated muscle, the contractile pressure response was faster, d-tubocurarine sensitivity was greater, d-tubocurarine blockade had a shorter duration, and fatigue was faster but not uniquely fast as compared with results for the external anal sphincter. These results suggest that the periurethral striated muscle has similar characteristics to certain fast twitch muscles. Effect of succinylcholine on the periurethral striated muscle and external anal sphincter was similar. Miniature end-plate potentials had large amplitudes which appeared to be correlated to the small diameter of the periurethral striated muscle fibers. Since only focally distributed miniature end-plate potentials could be found, true physiologically slow muscle fibers were absent.