In tracheal smooth muscle, we obtained quantitatively different force-velocity (F-V) curves at early (2 s) and late (8 s) stages of an isometric tetanus whose contraction time was 12 s. These were essentially two samples from a continuum of F-V curves operating between 0 and 12 s. The cross-bridge cycling velocity at 8 s was slower and less sensitive to external load change compared with that at 2 s. This is possibly due to the presence of two types of cross bridges with different F-V characteristics; at 2 s most of the bridges resemble a cycling type, whereas at 8 s there is a population of what Dillon et al. have called slowly cycling or latch type interactions. Another possibility is that, due to some intrinsic factors, the whole population of cross bridges gradually change their F-V characteristics. Functions a(t) and b(t) were obtained by applying load clamps at 1-s intervals throughout a tetanus (a and b are asymptote values derived from the F-V hyperbolic curves): a increased with time, b remained constant. Analysis suggested that a/b was a valid index of internal factors that affect shortening velocity of unloaded muscle, and it is progressively increased in value during contraction.