NONINVASIVE photolytic injury to targeted neuronal subpopulations in vivo causes unique, slowly progressive neuronal degeneration. To examine the mechanisms of degeneration toward development and transplantation studies, cytosolic calcium levels were measured in vitro from neocortical neurons after incorporation of photoactive nanospheres and laser-activated singlet oxygen production within lysosomes. Cytosolic calcium increased irreversibly, predominantly from extracellular sources through channel-mediated mechanisms and increased membrane porosity. Propidium iodide studies demonstrated gradual loss of membrane integrity over hours to days. The calcium channel blocker nimodipine, or calcium-free medium, partially protected neurons from calcium flux and cell death. Results suggest calcium-dependent and independent mechanisms of neuronal degeneration following singlet oxygen production.