1. Normall and chronically stimulated peroneus longus muscles of the cat''s hind limb were studied with respect to fibre size and staining properties for myofibrillar (myosin) adenosine triphosphatase (ATPase) and succinate dehydrogenase (SDH) activity. The intensity of staining for SDH activity was measured by microphotometry from the central portions of the muscle fibres (''core-SDH staining''). For comparison, histochemical properties were also studied in non-stimulated soleus muscles. 2. On account of the pH sensitivity of their myofibrillar ATPase, about 18% of the fibres in normal peroneus longus muscles were classified as type I, and about half of the remainder as IIA and IIB respectively. 3. In the normal peroneus longus muscles, the mean remainder of single muscle fibres generally varied between about 25 and 75 .mu.m, whereby the average size of type I < type II. 4. In the normal peroneus longus muscles the staining intensity for core SDH varied over a wide range. The average heaviness of staining was clearly ranked in the order type I > type IIA > type IIB. 5. Chronic stimulation was given to the deafferented common peroneal nerve by aid of a portable and remotely controlled mini-stimulator. The stimulation was delivered in ''tonic'' patterns (.gtoreq. 50% of total time taken up by activity) of ''fast'' (20 or 40 Hz) or ''slow'' (5 or 10 Hz) rates. 6. Prior to the period of long-term stimulation, the cats had been subjected to a dorsal rhizotomy and hemispinalization on the ipsilateral (left) side. In the absence of chronic stimulation, these operations had no evident effects on the sizes or staining properties of peroneus longus fibres. 7. After 8 weeks of treatment with tonic patterns of stimulation, the fibres of peroneus longus muscles clearly became more similar to each other with respect to their diameter as well as their staining for ATPase and SDH activity. With respect to ATPase staining, however, the chronically stimulated peroneus longus fibres had become more similar to non-stimulated soleus fibres than to non-stimulated type I fibres of peroneus longus. With respect to the staining for core SDH, the chronically stimulated fibres all became similar to normal IIA fibres of peroneus longus. The ''fast'' and ''slow'' patterns of chronic stimulation had the same effects on the staining properties. 8. Chronically stimulated peroneus longus muscles showed a decrease in fibre diameter which corresponded, roughly, to the concomitant decrease in muscle weight. The decrease in fibre size was significantly greater for the ''slow'' patterns than for the ''fast'' ones. 9. The results were discussed in relation to previously obtained physiological findings from the same muscles. The absence of an evident relationship between the pulse rate of chronic stimulation and the ATPase or SDH staining is in accordance with the findings that the present patterns of stimulation also all had the same effects on the isometric contractile speed and the contractile endurance of peroneus longus. The finding that the ''slow'' patterns caused a greater decrease of fibre size than the ''fast'' patterns is in accordance with differences between the effects of these patterns on contractile force. The relative decrease in cross-sectional area was, however, less than the relative decrease in maximum tetanic force.
