Dendrites of CA1 pyramidal neurons in mature rat hippocampal slices were exposed to different levels of synaptic activation. In some slices, synaptic transmission was blocked with glutamate receptor antagonists, sodium and calcium channel blockers and/or a nominally calcium-free medium with high magnesium. In other slices, synapses were activated with low-frequency control stimulation or repeated tetanic stimulation. In slices with blocked synaptic transmission, dendrites were spinier than in either of the activated states. Thus, mature neurons can increase their numbers of spines, possibly compensating for lost synaptic activity.