Calcium/calmodulin-dependent protein kinase II (CaMK) and p42 mitogen- activated protein kinase (MAPK) are enriched in neurons and possess the capacity to become persistently active, or autonomous, following removal of the activating stimulus. Since persistent kinase activation may be a mechanism for information storage, we have used primary cultures of cortical neurons to investigate whether kinase autonomy can be triggered by bursts of spontaneous synaptic activity. We and others have found that both these kinases respond to synaptic stimulation, but differ markedly in their kinetics of activation and inactivation, as well as in their sensitivity to NMDA receptor blockade. While 90% of maximal CaMK activation was observed after only 10 sec of synaptic bursting, MAPK activity was unaffected at this early time and rose to only 30% of maximal after 2 min of stimulation. Following blockade of synaptic stimulation, CaMK activity decreased by 50% in 10–30 sec, while MAPK activity decayed by 50% within 6–10 min. Although MAPK exhibited relatively slow activation, short periods of synaptic activity could trigger the MAPK activation process, which persisted in the absence of synaptic stimulation. Comparison of the effect of NMDA receptor blockade on synaptic activation of these kinases revealed that CaMK activity is preferentially suppressed. As previous immunocytochemical studies indicate that CaMK is concentrated in dendritic processes in the vicinity of synapses, we measured synaptic calcium transients in fine dendritic processes (approximately 1 microns diameter) to assess their sensitivity to NMDA receptor blockade. Calcium transients in these fine processes were reduced by up to 90% by NMDA receptor blockade, possibly accounting for the profound sensitivity of CaMK to this treatment. The sharp contrast between the regulation of CaMK and MAPK by synaptic activity indicates that they may mediate neuronal responses to different patterns of afferent stimulation. The relatively slow activation and inactivation of MAPK suggests that it may be able to integrate information from multiple, infrequent bursts of synaptic activity.