In many cells, organelle translocation requires structural integrity of microtubules [2,6,11,14, 5]. We have found that extracellular application of the antimicrotubular drug colchicine, which was expected to enter cells simply by diffusion, was ineffective at blocking pigment granule migration (PGM) inside fly photoreceptors. However, illumination of these receptors in the presence of colchicine resulted in complete PGM block. From a combined study using microphotometry, microfluorometry and intracellular recordings, we inferred that light permeabilized the cell to the drug. This hypothesis was supported by the observation that illumination caused the photoreceptor to take up an inert dye. The results show that target cells optically selected from a homogeneous population can be induced to take up an agent from the extracellular fluid. Remarkably, this process of light-induced drug uptake does not affect cell viability.