The purpose of the study was to note whether insulin-like growth factor (IGF) immunoreactivity increased after eccentric contractions. IGF immunoreactivity in the rat tibialis anterior muscle was measured on 5 successive days (4-5 rats/group, n = 28) after an acute bout of 192 eccentric contractions elicited by electrical stimulation. The muscle tissue sections were immunocytochemically processed with rabbit anti-human IGF-I serum. Immunoreactivity was analyzed with videomicroscopy and computer-aided image processing. Four days after eccentric contractions, IGF immunoreactivity was significantly higher than control [0.081 +/- 0.073 (SD) absorbance at 480 nm vs. 0.026 +/- 0.018; P < 0.05]. The increases in IGF-I immunoreactivity were mostly within the muscle fibers. These results suggest that an acute bout of eccentric exercise increases IGF-I immunoreactivity in rat type II muscle 4 days postexercise.