Hippocampal neurons exhibit periodically recurring growth cone-like structures, referred to as "waves," that emerge at the base of neurites and travel distally to the tip. As a wave nears the tip, the neurite undergoes retraction, and when it reaches the tip, the neurite undergoes a burst of growth. At 1 day in culture, during early axon outgrowth, axons undergo an average 7.5-microm retraction immediately preceding wave arrival at the tip followed by 12-microm growth immediately after arrival (an average net growth of 4.5 microm). In branched axons, waves often selectively travel down one branch or the other. Growth selectively occurs in the branch chosen by the wave. In dendrites, which grow much slower on average, wave-associated retractions are much greater, resulting in less net growth. In the presence of Brefeldin A, which disrupts membrane traffic through the Golgi apparatus and leads to retraction of the axon, axonal waves continue to be associated with both growth spurts and retractions. The magnitude of the growth spurts is not significantly different from untreated axons, but wave-associated retractions are significantly increased. The close association between waves and cyclical elongation suggests that waves may act to bring about this pattern of growth. Our results also show that modulation of regularly occurring retraction phases plays a prominent role in determining average outgrowth rates.