Sprague Dawley rats ( n=6, age 87 days) were injected 2x daily for 43 days with P-aminopropionitrile (BAPN, 333 mg*kg-1*day-1 i.p.) which inhibits the enzyme, lysyl oxidase, responsible for collagen cross-links. Age-matched saline-injected rats served as controls (C, n=6). Isometric forces before dorsiflexion (a = 30000 degrees -s*-2, ankle position 90 degrees to 40 degrees) with repeated stretches (20) of active plantar flexor muscles at 80 Hz (contraction time 1.1 s, rest periods 3 min) were measured. Isometric forces were measured at 90 degrees, before and 1 hr after the stretches at 5, 10, 20, 40, 60 and 80 Hz. Relative weights of soleus, plantaris and gastrocnemius muscles were not different between groups. Significant reductions in collagen cross-links, pyridinoline, (mol/mol collagen) were found in tendon (22.9%), plantaris (17.1%), and soleus (7.4%) with no changes in collagen content (hydroxyproline) as determined by HPLC. In gastrocnemius medialis, pyridinoline cross-links were 4.4% lower but this did not pass significance (P = 0.2). Groups had similar isometric forces at 5, 10, 20, 40, 60 and 80 Hz before the stretches. Isometric force deficits that developed followed similar courses for BAPN- and C-rats. Before the last stretch, deficits were 51.1 % +/- 2.4% (C) and 54.7 % +/- 4.6% (BAPN). After 1 hr of rest following the stretches, deficits were similar at 80, 60, 40 and 20 Hz but were 26% and 29% larger at 10 and 5 Hz in BAPN-rats (P < 0.05). Isometric force deficits after active stretches of rat plantar flexor muscles with reduced collagen cross-links, pyridinoline, were larger only at low stimulation frequencies.