Evidence has accumulated that synaptic activation enhances the incorporation of labeled uridine into Aplysia neuronal RNA. This paper reports evidence which indicates that the enhanced synthesis is: (1) linear with time, (2) is correlated with duration and, to a lesser degree, with frequency of stimulation, and (3) is reversed by the cessation of the stimulus. Efforts to characterize further the RNA which contains the increased label suggest that it is one of the less methylated species and that its hybridization characteristics differ from the RNA of the unstimulated neuron. These results, and others, lead us to conclude that any neuronal plastic changes, whose onset occurs in much less than 1 h, are unlikely to involve changes at the transcriptional level.