Morphological, histochemical and ultrastructural characteristics of intrafusal fibre types were studied in rat muscle spindles. The existence of three intrafusal fibre types, namely the typical bag, the intermediate bag and the chain fibres was confirmed. Intrafusal fibres differ in diameter, length and number of nuclei in the equatorial zone. Histochemically, typical bag fibres exhibit both alkali- and acid-stable ATPase activity and low SDH activity. Intermediate bag fibres possess low alkali-stable ATPase activity; after acid-preincubation, however, they have low activity only in the juxtaequatorial region, whereas in the polar zones they exhibit high acid-stable ATPase activity. The SDH activity varies from moderate to high. The chain fibres exhibit high alkali-stable and low acid-stable ATPase and high SDH activity in the extensor digitorum longus muscle, whereas in the soleus muscle the acid-stable ATPase activity varies from a low one to a high one, either among individual chain fibres in one spindle, and/or repeatedly along the fibre length. Since there are regional differences in morphological characteristics and in staining properties of intrafusal fibres, a reliable identification of intrafusal fibre types can only be achieved by an analysis of serial sections.