In order to examine the effects of activity on spine production and/or maintenance in the cerebral cortex, we have compared the number of dendritic spines on pyramidal neurons in slices of P0 mouse somatosensory cortex maintained in organotypic slice cultures under conditions that altered basal levels of spontaneous electrical activity. Cultures chronically exposed to 100 microM picrotoxin (PTX) for 14 days exhibited significantly elevated levels of electrical activity when compared to neurons in control cultures. Pyramidal neurons raised in the presence of PTX showed significantly higher densities of dendritic spines on primary apical, secondary apical, and secondary basal dendrites when compared to control cultures. The PTX-induced increase in spine density was dose dependent and appeared to saturate at 100 microM. Cultures exhibiting little or no spontaneous activity, as a result of growth in a combination of PTX and tetrodotoxin (TTX), showed significantly fewer dendritic spines compared to cultures maintained in PTX alone. These results demonstrate that the density of spines on layers V and VI pyramidal neurons can be modulated by growth conditions that alter the levels of spontaneous electrical activity.