Dynamic rearrangements of axon terminal arbors may be critical for establishing appropriate connections in the developing nervous system. Here, the changes in complex retinal axon arbors in the tecta of live Xenopus larvae were followed during the formation of the topographic retinotectal projection. Three-dimensional reconstructions of terminal arbors made with a confocal microscope at hourly intervals revealed rapid remodeling of arbor extensions. Shorter branches were extended and retracted very rapidly, suggesting that they probe the environment for the optimal sites to form stable branches. About 27% of longer branches were present throughout the entire observation period and may be sites of stabilized synaptic contacts. Treatment of the animals to block postsynaptic activity resulted in increased rates of arbor rearrangements, which may coincide with decreased synapse stability. These studies reveal the dynamic behavior of nerve arbors and provide estimates for the lifetimes of retinotectal branches.