Rat respiratory muscles underwent considerable changes in histochemical fiber type profile in response to hypo- and hyperthyroidism. Hypothyroidism increased the proportion of type 1 slow oxidative fibers in diaphragm and to a lesser extent in intercostal muscles. Hyperthyroidism resulted in a decreased proportion of type 1 fibers in diaphragm and intercostals. These changes were broadly comparable to those reported previously in rat limb muscles. In normal rat respiratory muscles, the type 1 fibers were characterized by very high levels of .beta.-hydroxybutyrate dehydrogenase which was thought to contribute to the fatigue-resistance of these muscles. The type 2B fast glycolytic fibers, and to a lesser extent type 2A fast oxidative fibers, contained high levels of mitochondrial .alpha.-glycerophosphate dehydrogenase, an enzyme known to be specifically affected in dysthyroid states. The implications of the observed changes in fiber type profile with respect to the oxidative metabolism of rat respiratory muscles are discussed.