Overloading of rat plantaris muscles was produced by aseptic ablation of the synergists. The morphological changes occurring after 1 or 2 weeks were investigated at the light and electron microscopical level in the distal tendon of the plantaris and at the myotendinous junction. Sham-operated rats were prepared as controls. In the tendon, quiescent fibrocytes were replaced by activated fibroblasts displaying a vesicular nucleus with prominent nucleoli and an outstanding increase in cytomembranes, particularly the rough endoplasmic reticulum and the Golgi complex. The plasmalemma of the fibroblasts was modified by the presence of caveolae and the surbsurface cytoplasm contained many membrane-bound vacuoles. In the tendon, the collagen bundles were disrupted, resulting in the formation of empty longitudinally oriented spaces; in these spaces, as in the pericapillary areas, no inflammatory cells were observed. At the myotendinous junction, fibroblast activation was consistently observed in both control and overloaded specimens. At this level, the sarcolemma of the finger-like projections of muscle fibres presented many caveolae close to clusters of large subsurface vacuoles. These observations indicate that, at the beginning of the compensatory hypertrophy, the adaptative changes to overloading include a non-inflammatory reaction of the tendon characterized by enhanced collagen synthesis and intensive membrane renewal and recycling. From the mechanical point of view this reaction can impair the tendon resistance to stretch. At the myotendinous junction the increased membrane turnover of the sarcolemma and the fibroblast activation can be considered permanent phenomena consequent to the increased stress exerted upon the interface connecting the contractile apparatus to the stroma.(ABSTRACT TRUNCATED AT 250 WORDS)