We describe a new, simple and reliable technique to fill molluscan neurones from their cut axons with sufficient fluorescent dye for photoinactivation experiments. The fluorescent dye 5(6)-carboxyfluorescein (5-CF) travels quickly up the nerves of the gastropod mollusc, Lymnaea stagnalis into the buccal ganglia and fills the cell bodies in 1-3 h. 5-CF filled neurones can be located in the intact ganglia with low intensity blue light. Impalement shows that they are alive and show normal resting, action and synaptic potentials. Intense laser light (wavelength 442 nm, intensity 0.5 MW.m-2) kills all the 5-CF filled cells in less than 5 min in laboratory reared snails. Unstained neurones are not killed. 5-CF fills neurones quicker than Lucifer yellow (LY) when the dye is applied axonally. Neurones stained with Lucifer yellow do not contain sufficient dye to be killed with 5 min laser illumination, but this irradiation reduces the membrane resistance to less than 25%.