The underlying mechanism of stretch-induced delayed force increase (stretch activation) of activated muscles is unknown. To assess the molecular correlate of this phenomenon, we measured stretch activation of single, Ca2+-activated skinned muscle fibres from rat, rabbit and the human and analysed their myosin heavy chain complement by SDS gradient gel electrophoresis. Stretch activation kinetics was found to be closely correlated with the myosin heavy chain isoform complement (I, IIa, IId/x and IIb). In hybrid fibres containing two myosin heavy chain isoforms (especially IId and IIb), the kinetics of stretch activation depended on the percentage distribution of the two isoforms. Muscle fibres of the same type but originating from different mammalian species exhibited similar kinetics of stretch activation. Considering the differing unloaded shortening velocities of these fibres, the time-limiting factors for stretch activation and unloaded shortening velocity appear not to be the same. The stretch activation kinetics of the fibre types IIB, IID and IIA more likely seemed to follow a Normal Gaussian distribution than that of type I fibres. Several type I fibres had extraordinarily slow kinetics. This observation corroborates biochemical data indicating the possible existence of more than one slow myosin heavy chain isoform.