Muscle biopsy specimens from the myotonic goat, an animal model of heritable myotonia, were examined histochemically and by electron microscopy. After Periodic acid-Schiff (PAS) staining with diastase digestion, there was increased PAS-positive material within myotonic goat fibers, as compared with those of normal goats. Myotonic muscle stained with alizarin red S, a histochemical stain for calcium, also had an increased staining reaction when compared with muscle from normal goats. Several ultrastructural abnormalities were found in myotonic goat muscle using routine osmium and uranyl acetate staining. These included increased density of the t-tubules, electron-dense material within t-tubules, proliferation and dilatation of sarcotubular elements, and abnormal mitochondria in the myotonic biopsy specimens. To further study muscle ultrastructure, ruthenium red and lanthanum were used as electron microscopic stains with specificity for membranes. There was increased density of the sarcolemma and t-tubules in myotonic muscle stained with ruthenium red as compared to normal, and lanthanum produced a darker staining reaction of the myotonic goat sarcolemma. The histochemical and ultrastructural differences between normal and myotonic goat muscle were interpreted to be consistent with a morphologic basis for the abnormal contraction-relaxation properties characteristic of myotonia.