We have examined the influence of amino acids on chicken embryonic dorsal root ganglion (DRG) neurons in vitro and have found that serine has a marked impact on the morphology of the developing neurites. In cultures supplemented with L-serine (but not D-serine) at micromolar concentrations (10-200 microM), the length of the neurites was increased by up to 100% and they were seen to develop a more complex branching pattern. These effects of L-serine were found to be concentration-dependent and stereospecific and were observed on several different substrata such as laminin, Ng-CAM and axonin-1. Similar observations were also made in the case of embryonic retinal explants, while the addition of non-essential amino acids other than L-serine to DRG neurons was found to have no effect. We conclude that, although belonging to the group of non-essential amino acids and not a recognized neurotransmitter, L-serine is an important factor for the morphological differentiation of neurons in vitro.