1. The velocity of unloaded shortening (V0), the myofibrillar ATPase activity and the immunoreactivity to two monoclonal antibodies (A1 and A2) that were raised against the myosin heavy chains were studied in single fibres of the anterior tibialis muscle of Rana temporaria. V0 was recorded for the fibre as a whole using the slack-test method. Myofibrillar ATPase activity was determined by means of a quantitative histochemical technique. 2. A highly significant, direct relationship was found to exist between V0 and the myofibrillar ATPase activity recorded in the same single fibres. Both V0 and the myofibrillar ATPase activity changed in proportion to the cross-sectional area of the fibres. 3. Muscle fibres that had first been characterized with respect to V0 and myofibrillar ATPase activity were exposed to monoclonal antibodies A1 and A2. Thin fibres, having relatively low V0 and low myofibrillar ATPase activity, reacted preferentially with A1. Thick fibres, on the other hand, exhibiting relatively high V0 and high myofibrillar ATPase activity, were preferentially stained by A2. A third category of fibres reacted with both A1 and A2. The results support the view that the variability in shortening velocity and myofibrillar ATPase activity that exists among twitch fibres in frog skeletal muscle is based on differences in myosin heavy-chain composition. 4. Attempts were made to elucidate further the previous observation (Edman, Reggiani & te Kronnie, 1985) that the velocity of unloaded shortening (V0) differs along the length of individual muscle fibres. To this end discrete segments (0.5-0.7 mm in length) of intact fibres were delineated by opaque markers of hair that were placed on the fibre surface. The change in length between two adjacent markers (one segment) was recorded photo-electrically while the fibre was released to shorten against a very small load between 2.2 and 2.0 micron sarcomere lengths. In the majority of fibres (eight out of eleven preparations), V0 and myofibrillar ATPase activity exhibited similar patterns of variation along the fibre. Pooled data from thirty-three segments of twelve fibres showed a positive correlation between V0 and myofibrillar ATPase activity (P less than 0.05). 5. The possibility was explored that the myosin isoform composition might vary along the length of an individual muscle fibre. For this purpose bundles of fibres were cross-sectioned at 0.5-1 mm intervals along their entire length and the reactivity to monoclonal antibody A2 was tested at each location.(ABSTRACT TRUNCATED AT 400 WORDS)