1. Cultured rat hippocampal neurons were loaded with the Ca2+ indicator fura-2 through micropipettes and visualized with an inverted microscope equipped with a high power objective and a cooled CCD camera. The responses of dendritic spines and their parent dendrites to stimuli which evoke a rise of [Ca2+]i were monitored. 2. NMDA caused a rapid and transient rise in [Ca2+]i, which was more evident in the spine than in the parent dendrite. The recovery in both compartments had the same time course, and was dependent on normal [Na+]o. 3. Application of alpha-latrotoxin, which causes release of neurotransmitters from terminals, produced a rise of [Ca2+]i in the dendritic spines, more than in their parent dendrites. Prolonged exposure to the drug eliminated the spine/dendrite disparity. 4. The presence of voltage-gated calcium channels in dendritic spines is indicated by the enhanced calcium rise in spines rather than dendrites of cells depolarized by either intracellular current injection or by raising [K+]o. This rise was attenuated by nifedipine or verapamil, both L-type channel blockers. 5. It is suggested that the dendritic spine constitutes an independent calcium compartment that is closely linked to the parent dendrite.