Fibre length, fibre angle and muscle length were quantified for rat m. gastrocnemius medialis with the muscle passive as well as fully activated during isometric contraction. This was done with the muscle in situ still attached with intact origin and insertion as well as with the calcaneus cut for simultaneous force measurements. Comparison of muscle lengths in maximal plantar and dorsal flexion with the physiological length range of the muscle, as defined by the limits of the length force relation, indicated that approximately the lower 75% of this range may be used between the extreme ankle angles, while the knee is kept at 90° of flexion. It is likely that simultaneous knee extentension would take the muscle through the remainder of its physiological length range. During isometric contractions at the level of the muscle, fibres shorten and fibre angles increase (with values exceeding 12 % and 45 % respectively at short muscle lengths). At short lengths fibre angle may reach values exceeding 40°, thereby creating sizable differences between force exerted by the muscle and that of its fibres. Changes of fibre length and fibre angle increase with decreasing muscle length and are ascribed to compliance effects of the aponeuroses above muscle equilibrium length while below muscle equilibrium length a taking up of slack present in these structures occurs prior to these compliance effects. During isometric contractions at the level of the muscle-tendon complex work will be performed by the muscle on the achilles tendon. This work was estimated from tendon length-force characteristics. Its peak value does not exceed 1.35 mJ for any of the muscles at anv length, which is small ( < 2 % ) relative to estimated total energv expenditure of the isometric contractions.