The purpose of this study was to investigate the enhancement of shortening and of the velocity of shortening during repeated incompletely fused isotonic tetanic contractions. The medial gastrocnemius muscle of anesthetized rats was isolated in situ and the motor nerve stimulated with supramaximal pulses, 50 micros duration, at optimal length. Estimated maximal velocity of shortening (V(max)) was 210 +/- 6 mm x s(-1) (mean +/- SEM). Repeated incompletely fused tetanic contractions (three pulses at 80 Hz) resulted in initial shortening which was 1.5 +/- 0.1 mm, and this increased to 2.7 +/- 0.2 mm after 7 s of 4 s(-1) contractions. Peak velocity of shortening for intermittent 80 Hz stimulation increased from 60.5 +/- 4 mm x s(-1) to 91.8 +/- plus minus 6 mm x s(-1). The initial velocity of shortening for 80 Hz stimulation was substantially less than the velocity of shortening observed with 200 Hz stimulation, but increased to 72 +/- 3% of the load-specific value for 200 Hz stimulation. Myosin regulatory light chain phosphorylation increased from 11.1 +/- 1.5% at rest to 32.9 +/- 5.4% after 4 s of intermittent 80 Hz stimulation and 50.4 +/- 8.8% after 7 s ( P<0.01). The ascending limb of the length-force relationship was determined with tetanic contractions, 200 Hz for 100 ms. At the length corresponding to peak shortening after 7 s of repeated 80 Hz contractions, the maximal isometric force was five times greater than the isotonic load. The rate of phosphorylation was sustained from 4 to 7 s, but the rate of increase in shortening and velocity decreased. The slower rate of change in shortening and velocity may be due to approaching maximal velocity for this short duration of contraction, but is not due to slowing of the rate of phosphorylation of the myosin regulatory light chains.