The structure of individually identified neuromuscular junctions (NMJs) in mouse lateral gastrocnemius (LG) muscles was studied on 2 or more occasions over 3-6 months. Presynaptic motor nerve terminals and their underlying acetylcholine receptors were stained in living animals with the fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide) and tetramethylrhodamine isothiocyanate-conjugated alpha-bungarotoxin (R alpha BTX), respectively, and visualized by video-enhanced fluorescence microscopy. The overall shape of most NMJs changed very little over this time except for enlargement of some junctions attributable to growth of the animals. A few junctions did, however, change appreciably: over 3-6 months about 15% underwent modifications such as additions to, or losses from, their original configuration. The frequency and extent of changes in LG NMJs were substantially less than in a similar study of NMJs from mouse soleus (Wigston, 1989). These findings, together with those from other laboratories, indicate a correlation between the extent of NMJ remodeling and the fiber-type composition of a muscle: NMJs in muscles consisting of predominantly fast-twitch myofibers appear to undergo less remodeling than NMJs in muscles containing a substantial fraction of slow-twitch fibers. Since fast- and slow-twitch muscles and their motoneurons exhibit strikingly different patterns of electrical activity, these findings suggest that structural remodeling at mammalian NMJs may depend on the amount of impulse activity experienced by motoneurons, their target muscle, or individual synaptic terminals.