The fluorescent dye 4-(4-diethylaminostyryl)-N-methylpyridinium iodide was used as a vital stain to study remodelling of motor nerve terminals in sartorius muscles of living frogs (Rana pipiens). Identified terminals were observed twice in vivo at intervals of 87-192 days. After the second observation, muscles were fixed and stained with the nitroblue tetrazolium method for nerve terminals and with cholinesterase stain. Observations were made of 243 junctions in 26 frogs. Most nerve terminals grew during the observation interval, with an average increase in total terminal length of 29%. This growth involved substantial remodelling. Within single junctions, the change in size was the net result of differing degrees of growth or shrinkage in individual nerve terminal branches. At least one new terminal branch appeared in 25% of the junctions. Terminal retraction was also common, with branch shortening seen in 60% of junctions and the complete disappearance of a branch in 12%. In one case the original axonal input retracted completely and the junction was partially reinnervated by a terminal sprout from a junction on an adjacent fibre. Some discrepancies between histological and in vivo observations of remodelling were noted. These observations confirm that frog neuromuscular junctions are highly dynamic synapses, subject to profound structural remodelling throughout adult life.