Identified neuromuscular junctions were followed in the sternomastoid muscle of living mice for several months by repeated staining with the fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide (4- Di-2-ASP; Magrassi et al., 1987). Overall terminal growth occurred at many endplates; however, the branching pattern of presynaptic arbors was largely unchanged, even after several months. The absence of significant remodeling over time was not a result of dye-staining, since sprouting was readily induced at residual motor endings by partial denervation. We conclude that--apart from overall growth--most neuromuscular junctions in the adult mouse are stable over intervals that represent a significant fraction of the animal's lifetime.